How to make this cleric/warlock idea viable

[Dualight]

This idea came to me in response to something i saw in the list of suggested deities for the Order domain cleric in the Guildmasters' Guide to Ravnica. I noticed that Asmodeus was one of the suggested deities, and remembered that Asmodeus is also a valid (and suggested) Archiend patron for a warlock.

This gave me the idea of a charactger that is contracted to Asmodeus as both a warlock and cleric. However, I keep struggling with how to express the concept properly, without making a non-functional character.

So my question to whoever is willing to answer: How would you go about building a Archfiend warlock/Order cleric multiclass that has to end at 10/10, preferably levelling the two classes mostly evenly(so something like Warlock 1 -> Cleric 1-. warlock 2), although letting them diverge at points if that makes the build more playable is probably a good idea.

I stalled out at taking the Book of Ancient Secrets invocation to get all cleric rituals (up to 5th level) effectively always prepared, and rat realising that the capstone would be Fiendish Resilience or Divine intervention, depending on which class hits 10 last.

I am well aware that this is a severely underoptimised concept, but I am curious how to make it work. I am not familiar enough with clerics or warlocks to tell what synergies/anti-synergies there are. (note of disclosure: I have never played a character at a level higher than 7th outside rare one-shots, so I am overall unfamiliar with higher-level play in general, which is where I expect this concept to either fall behind hard, or conversely to finally get going, but I have too little familiarity with actual play to tell.)

[dehro]

from what I can tell, it would indeed fall behind hard.. it's rather MAD, especially with the restrictions you impose on it... it can still be fun to play the concept if you don't worry about lagging behind others and they don't worry about carrying a part of your weight.
much depends on playstyle anyway.
that said, I shall leave the tweaking and numbercrunching to those that are better than me at it (most people, really, lol)

[Arcomage]

I am well aware that this is a severely underoptimised concept, but I am curious how to make it work. I am not familiar enough with clerics or warlocks to tell what synergies/anti-synergies there are. (note of disclosure: I have never played a character at a level higher than 7th outside rare one-shots, so I am overall unfamiliar with higher-level play in general, which is where I expect this concept to either fall behind hard, or conversely to finally get going, but I have too little familiarity with actual play to tell.)

The good: Since you're getting armor proficiency from the Order Cleric, you're not really behind on your defenses even if you have to split your attention between Wisdom and Charisma, but do probably want some minor investment in either Strength or Dexterity to wear heavy or medium armor effectively. Dark One's Blessing is a fairly good passive ability for using the likes of Spirit Guardians and can give you the toughness you'd need to walk into melee range where a lot of the Cleric spell list in general shines, and the Warlock side can offer you some very solid cantrips (Toll the Dead and Eldritch Blast, mainly) to cover your ranged combat needs while throwing around bonus-action Commands.

The bad: You're going to want to focus on one over the other in terms of spellcasting - a 10/10 level split is kind of painful since it effectively makes you a half-caster at both ends, and either way you do it one of your casting stats is probably going to lag behind. You might also be able to squeeze some kind of melee build out of the concept using Pact of the Blade and the built-in Order cleric melee benefits, but the investment in Strength or Dexterity that requires will hurt your saving throw DCs on both ends. A character that plays mostly as a Warlock can get away with low Wisdom and snipe some good Cleric buffs, while a mostly-Cleric primarily wants utility out of the Warlock side - Chain, Tome and Talisman all have good things going for them in such a build.

The ugly: Voice of Authority, the defining feature for Order Clerics, really wants you to be casting ally-targeted spells whenever you can, and the Warlock spell list is very bad at offering any. Your Concentration is a heavy limitation on both ends of the multiclass, and you likely won't be able to invest in Constitution saves enough to make those checks very consistent.

Overall, I would suggest picking up War Caster fairly soon - it'll help with your concentration, it lets you use a shield with your Warlock spells, and can give you an opportunity attack worth using - Toll the Dead might be a good pick for that purpose. Your combat style as a whole probably needs to focus on either Warlock or Cleric side first - getting the multiclass in early isn't too bad for 1 level, but after that you really want to take a class to at least level 5 as quickly as you can. As a Warlock, you'd raise Charisma first, use Eldritch Blast, summoning spells and the blasting spells from your Fiend pact to harry opponents from the mid-range and disrupt them with Command while possibly maintaining a Bless from your Cleric levels (which also grants an ally a free attack when you first cast it). If going for Cleric first, you'd probably want to raise Wisdom instead, take to the front lines with whatever spells you can get to help out allies, and use Armor of Agathys to make life very unpleasant for anyone with the bright idea to take out the support first.

At higher levels, you can more seamlessly swap between those styles as required. You probably want to replace your Divine Strike with Blessed Strikes if it's allowed because it also applies to your Warlock cantrips. As long as you're aware that this whole thing is kind of suboptimal (and your group doesn't expect you to be super optimized), I think it can be a perfectly valid and functional build, though.

[Xetheral]

Eldritch Blast with the Agonizing Blast invocation has the virtue of being combat-effective all on its own. Therefore, while most 10/10 splits will be terrible, Warlock 10/[Anything] 10 will at least be able to put out sufficient damage to be useful.

Therefore the trick isn't making a Fiend Warlock 10/Order Cleric 10 effective, the trick is making it fun, as simply spamming Eldritch Blast each turn can get rather repetitive. Here's what I would suggest:

Race/Ability Scores/Background: Mountain Dwarf, putting your ability bonuses in Wisdom and Charisma. Starting stats: 10/8/14/8/17/17. For Background, take Far Traveller for Insight and Perception proficiency.

Fiend Warlock 1: Use Hex and Eldritch Blast in combat as any other starting Warlock. Your AC is terrible, so use full cover extensively. For your class skills, take Deception and Intimidation.

Fiend Warlock 1/Order Cleric 1: For combats where you have time to prepare ahead of time, use Bless instead of Hex. When combat starts without prep, stick to Hex so you don't lose a round of blasting. You now have heavy armor and shield proficiency, so get your hands on both ASAP to get your AC up to something survivable. For your Order Cleric bonus skill, take Persuasion to round out your social skills.

Fiend Warlock 2/Order Cleric 1: Get the Eldritch Blast invocation. For your other invocation, take Misty Visions.

Fiend Warlock 3/Order Cleric 1: Take Pact of the Chain. Your Imp is both thematic and a super-scout. Use Misty Visions to show the rest of your party what your Imp is seeing so that scouting is a whole-party affair.

Fiend Warlock 4/Order Cleric 1: Bump Wisdom and Charisma to 18 each.

Fiend Warlock 5/Order Cleric 1: You now have access to Fireball, and your combat options drastically improve as a result. Gift of the Ever Living Ones is worth considering for your invocation, but Voice of the Chain Master is decent too.

Fiend Warlock 5/Order Cleric 2-5: Focus on Bonus Action Cleric spells like Sanctuary, Spiritual Weapon, and Healing Word so that you can keep spamming Eldritch Blast every turn. Remember that with Voice of Authority, Healing Word also gives your ally a Reaction attack. Bump Charisma to 20 at Cleric 4.

Fiend Warlock 5/Order Cleric 6: You get Embodiment of Law at this level, which lets you quicken Enchantment spells. Use it to cast (or recast) Bless as your main concentration spell using a bonus action. An occasional quickened Hold Person or Command can also be worthwhile.

Higher Levels: You've got almost all of your combat options at this point, so fill out your spell list with utility options. At Warlock 9 Synaptic Static is worth picking up for use with Embodiment of Law in tough fights.

All in all you're a decent damage dealer and face, with enough unused spell picks to be a versatile utility caster. You're not going to top any list of optimized characters, but this is definitely playable and hopefully will have enough round-to-round Bonus Action options to give you meaningful choices in combat alongside your Eldritch Blast spam.

[Sorinth]

Half-joking but use Scorching Ray but send one of the beams at an ally to trigger Voice of Authority.

More seriously I'd probably go melee using GFB/BB as the main attacks and using Armor of Agathys and Spirit Guardians. At first glance it seems extremely MAD, but you can focus on spells that don't require attacks/saves and thereby pump str/dex as your attack stat. Might even be worth going Tomelock to grab Shillelagh so that you can attack with Charisma and become less MAD.

[Danielqueue1]

Animate dead is a cleric spell. Cast it using warlock spell slots. Short rest to get them back. Buff allies or your skeletons with cleric spells, then cantrip it up with bonus action magic stone handing them off to your skeletons to throw on their turns, then action Eldritch blast. Also go pact of the chain and have your familiar deliver the magic stones (touch range spell) so you and your party won't get stuck in any AoEs the enemy sends at your skeletons. From Asmodeus to you to an imp to minions. A clear and ORDERLY Heirarchy. Maybe even have your spiritual weapon be a "chain of command" (for times when you don't have skeletons handy for your bonus action.

-edit check with your DM If "follow me and attack anything I attack." Is a good ongoing order for them to follow or if your DM is extra generous see if you can have your familiar use their bonus action to command them. (Order them to follow your familiar's orders and delegate)

[Rukelnikov]

Eldritch Blast with the Agonizing Blast invocation has the virtue of being combat-effective all on its own. Therefore, while most 10/10 splits will be terrible, Warlock 10/[Anything] 10 will at least be able to put out sufficient damage to be useful.

This.

You are a Warlock, at level 2 take Agonizing Blast, done, you can do whatever you want with the rest of your build, you'll do fine in combat.

[Dualight]

Thank you to everyone who replied. Thanks to all the feedback, this has gone from a silly idea I felt like i should look into one day, perhaps for an NPC(whose exact abilities wouldn't matter), to a concept I actually want to play someday. Now I am once again stuck with the usual problem of not having enough campaigns for all the characters I want to play, which is honestly not that bad of a problem to have.

To pick out a specific suggestion that I wouldn't have considered by myself, is how fitting Pact of the Chain is with this concept. When it comes to the pact boons, my mind tends to go weapon attacker=Blade, caster=Tome, and completely forgetting that Chain exists.

So once again, thank you to all who responded, as well as to all who might respond later. I enjoyed reading the responses so far.

EDIT: I wonder what other combinations are doable with the same concept(Deity=Patron)? the first to come to mind is Vecna(Knowledge & Undead/Undying), but i am certain that there are others. Tiamat(Trickery & Archfiend)? I am nowhere near well-versed enough in D&D lore to come up with more.