Please rate my first built-from-scratch homebrew monster.

[BeastlordHiro]

Like the title says its my first creation and I'd like to know if/how bad I screwed it up lol

[JNAProductions]

It's got a lot of immunities, and resistances.

On the whole, I'd say it doesn't look like a fun monster to face. It's damage output is ridiculously anemic (7.5 DPR with 100% accuracy, at +5 to-hit) though it has a decent chunk of HP. But Mind Rot means that casters will hate it, while those without slots will just plow through it.

So, what are you trying to accomplish with this monster? What's your goal?

[BeastlordHiro]

Well I'm thinking it'll be a specialized version of a magically constructed footsoldier type enemy that one of my bad guys uses. This one is designed to specifically take down any casters that may become a nuisance for their master.

[JNAProductions]

Well I'm thinking it'll be a specialized version of a magically constructed footsoldier type enemy that one of my bad guys uses. This one is designed to specifically take down any casters that may become a nuisance for their master.

Give it Counterspell. Or Dispel Magic. Once per rest, maybe.
Give it the ability to make anyone grappled by it subject to an Antimagic Field effect. (And make it decent at grappling too.)

Don't just give it the ability to boringly suck spell slots on each of its piddly hits. And I want to emphasize the "boringly" bit-that's the rule sin I see in this design. It's making spells boring, because they never get used.

[Vrock Bait]

Oh yes, and who gets to decide what spell slots in particular are lost? Can someone just lose four copies of Hold Portal or something?

[JNAProductions]

Oh yes, and who gets to decide what spell slots in particular are lost? Can someone just lose four copies of Hold Portal or something?

Not how 5E casting works, but that is something to clarify, yes.

[Breccia]

The most obvious compare and contrast is the mage-eating Helmed Horror, also CR 4.

Thing is, the HH has magic resistance, meaning it's more likely to take half damage from attacks that allow a save. Meanwhile, this monster has so many resists and immunities that it'll take half from nearly everything, and could take quarter if a save is allowed.

The HH does about double damage, but has half the hit points, which kind of events out...except it has no ranged attack, and this creature does. Helmed Horrors are also objectively easier to damage.

At level 4, there's not much a spellcaster has that will do a lot of damage. There's magic missile, eldritch blast, sacred flame and...that's about it.

I also looked at its overall numbers. If a 4th level fighter/thief/wizard/cleric fights one without magic weapons, even if the monster only attacks the fighter, it could do enough damage to drop him before the fight ends. The only recourse is for the wizard and cleric to throw spells to speed up the fight or heal, which the creature would consider a win considering its main purpose is to eat spell levels.

As others have said, the effect of being hit isn't fun. That could easily be more spell levels than they get back in a short rest. It might also drain paladin/ranger spells to 0 by accident.

And last but not least, I'm really worried when an evil boss monster brings out a couple of these on raised platforms and says "spam throwing spears at the cleric".

I would recommend toning that effect down. Perhaps an effect that suppresses their highest level spell slot, stacking, until the monster dies/the effect is removed/short rest/save ends?