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witchwood
2018-12-17, 06:50 AM
I'm designing a pirate themed campaign and am trying to develop a unique boss monster for the mid-point, utilizing the Dark Souls approach of having the monster change its tactics as the fight goes on. The party of 5 should be around 6th level by the time they hit this encounter and I'd appreciate any help balancing the numbers to make for a suitably epic fight. I'm also thinking about on giving the monster multiple rounds each turn so it doesn't get overwhelmed by the action economy as so many solo fights seem to, but again I'm not sure how to properly balance this. The fight itself takes place in a large, underwater cavern roughly 50x50 ft that is nonetheless drained of water for the fight itself. The monster is a Huge Hermit Crab using the magically enlarged and still alive undead head of a famous pirate captain as a shell. The head is missing its lower jaw but its eyes still move and it emits a low moaning sound as the creature fights.

The idea is to split the fight into 3 phases (or potentially less if the players are clever or lucky enough). In the first phase the crab fights from inside the skull, using its claws and some form of ranged attack (poison?), while it's shell grants it cover from attacks. Attacks that miss but roll high enough are marked as hitting the skull itself, which has significant damage reduction against most attacks and also has it's own separate health meter. In the (unlikely) occasion that the party deals sufficient damage to the skull to destroy it before the second phase is triggered, the fight immediately enters the third phase. Otherwise, once the party reduces the crab itself to 2/3 health, it retreats inside the skull, leaving only its legs exposed to continue moving with. In this second phase the crab itself is very difficult (but not impossible) to hit and it loses its melee attacks. Instead, the skull itself starts attacking with magical abilities such as rays and cone attacks from its eyes. If the crab is damaged enough to kill it during this phase the skull becomes immobile but continues fighting, albeit at a severe disadvantage now that it cant move. If the skull is instead destroyed the fight enters phase three, in which the crab itself becomes significantly harder to hit and much more mobile while losing its cover. The crab also becomes far more aggressive in this stage (possible represented by additional actions?) and the fight only ends when both the skull and the crab are both destroyed. I'm toying with the idea of using some kind of minion monsters along the main boss to divert the parties attention but I want the crab itself to obviously be presented as the main threat.

It's going to take me a while to determine the actual numbers the boss will use and I'd appreciate any general advice on how to make sure the fight a real challenge without rendering it impossible, as well as additional ideas to enhance the fight such as making the terrain difficult or setting it in low light. I've never designed my own boss monster before, let alone one this complex, so any and all help is welcome.

Unoriginal
2018-12-17, 07:15 AM
Well, my first advice would be to look at the Eidolon from the Mordenkainen's, for inspiration in the "two monsters in one" department. It's a spirit that can possess and animate giant statues, but both the spirit and the statue have separate statblocks

My second advice is that: make the crab and the skull separate monsters, give the skull an ability to protect the crab at the cost of mobility, and give them both Legendary actions. Also, give the Crab Lair actions.

How big do you want the crab to be? Is it a sapient being or a beast? Also, to help suggest some appropriate abilities, why are the PCs fighting it?

Helldin87
2018-12-17, 07:21 AM
Not entirely sure about the numbers but if you don't RP the boss like the giant crab from Moana you are missing an opportunity.

If I had to hazard a guess maybe start with some average damage output? For example if you are playing a game with arcane casters assume they have access to 2xD10 firebolt at lvl 6. A fighter should be able to do 2x2d8+4 at this level. None of this takes into account using a spell slot or a combat ability. So if a lowball player DPR is 12 per round lets arbitrarily make it 15 DPR to account for crits and spells. (fuzzy math is best math)

Party of 5 x 15 DPR=45 party DPR. I think this is pretty reasonable for 6th level players.

How long do you want each phase to last? I think at level 6 one round per phase is OK. Especially if you allow the bosscrab to act multiple times per round (probably should do so at multiple initiative counts).

Final answer: 135-160. Deadly but not impossible? A lot depends on your players and what they bring to a fight. If they are all min-maxed out the whazoo then more HP/boss abilities is better for sure.

Sounds like a cool fight with a lot of thought on it from your end. Have fun!