Hello tabletop peeps! I have completed a monster design for my first ever boss creature, and I wanted to get some quick feedback. This is all based on a previous post where I got ideas from y'all, here: https://tildes.net/~games.tabletop/1cjn/designing_my_first_dnd_boss_fight_with_a_giant_mimic_5x_lvl_5_characters

Players are a group of 5 lvl 5 characters, fairly magic heavy and lower hp overall.

The boss creature is a mimic that appears to be a large stone fountain with blood in it. If a PC touches it or stabs it, then they (or their weapon) will be grappled for the start of combat. At that point, the mimic will sprout 6 tentacles that each can travel up to 40ft from the main body and wield a weapon. All tentacles will operate on the same turn as the main body.

Healing Pool in the boss room

The boss room is a large room with a pool in it where blood is mixed with water that has flowed over a magic crystal to produce a healing solution. The idea is that the PCs will figure out that they can either heal themselves by dipping into the pool, prevent the boss from getting close to the pool, or that they can break the magic crystal to prevent the boss from healing. Several of the boss abilities reference this pool.

Mimic main body:

STR 16 (+3)
DEX 10 (+0)
CON 20 (+5)
INT 16 (+3)
WIS 12 (+1)
CHA 8 (-1)

HP 200
Main body loses 20hp if a tentacle dies

Armor Class: 13
Speed: 15 ft
Proficiency Bonus: 2
Damage Resistances: None
Immunities: Poison, Acid, Sleep, Charm

Abilities

Harden: Cannot take other actions, harden body to stone and increase AC to 18, as well as resistance to bludgeoning, piercing, slashing, cold, fire
Spew Blood: Begin charging up blood, PCs can see mouth direction. At the beginning of next turn, spew acidic blood in a 180 degree arc for 30 ft. Deals 3d6 acid damage, half damage on DEX 14 save. Mimic main body cannot do anything on its next turn as it recovers.
Chomp: +5 to hit. 1d12+3 piercing damage
Charge: If no arms are left, it will spew blood from the back and charge at a target. Increase speed to 45ft, and deal (+5 to hit) 4d6+3 bludgeoning damage to a target. Dex15 save for half damage.

Sword Tentacle (x3)

HP 20
Armor Class 13
Speed 30 ft
Damage Resistances: None
Damage Immunities: Poison, Acid

Abilities

Harden: Cannot take other actions, harden body to stone and increase AC to 18, as well as resistance to bludgeoning, piercing, slashing, cold, fire
Heal: If the healing pool is nearby, soak arm in the pool to heal it fully, and heal the main body for 15hp. Alternatively, resurrect a different defeated tentacle instead of healing self or the main body.
Slice: +5 to hit. 1d8+3 slashing damage

Shield Tentacle (x1)

HP 20
Armor Class 13
Speed 30 ft
Damage Resistances: None
Damage Immunities: Poison, Acid

Abilities

Harden: Cannot take other actions, harden body to stone and increase AC to 18, as well as resistance to bludgeoning, piercing, slashing, cold, fire
Heal: If the healing pool is nearby, soak arm in the pool to heal it fully, and heal the main body for 15hp. Alternatively, resurrect a different defeated tentacle instead of healing self or the main body.
Shield Bash: +5 to hit. 1d4 + 3 bludgeoning damage
Defend: As a reaction, give disadvantage to an attack when an ally 5ft away is attacked

Crossbow Tentacle (x1)

HP 20
Armor Class 13
Speed 30 ft
Damage Resistances: None
Damage Immunities: Poison, Acid

Abilities

Harden: Cannot take other actions, harden body to stone and increase AC to 18, as well as resistance to bludgeoning, piercing, slashing, cold, fire
Heal: If the healing pool is nearby, soak arm in the pool to heal it fully, and heal the main body for 15hp. Alternatively, resurrect a different defeated tentacle instead of healing self or the main body.
Heavy Crossbow shot: +2 to hit. 1d10 piercing damage

Handy Tentacle (x1)

HP 20
Armor Class 13
Speed 30 ft
Damage Resistances: None
Damage Immunities: Poison, Acid

Abilities

Harden: Cannot take other actions, harden body to stone and increase AC to 18, as well as resistance to bludgeoning, piercing, slashing, cold, fire. Cannot harden when grappling a PC.
Heal: If the healing pool is nearby, soak arm in the pool to heal it fully, and heal the main body for 15hp. Alternatively, resurrect a different defeated tentacle instead of healing self or the main body.
Grapple: +5 to hit. Grab a target, they can avoid it with a DC16 strength saving throw. If the target is successfully grappled, they can escape with a DC14 strength save at the start of their turn.
Squeeze: If a target is grappled, squeeze them for 1d6+3 bludgeoning damage.
Throw juice: If the healing pool is within range, scoop up a handful of purple juice and throw it at a skeleton to resurrect it.

Potential Adds

There are two skeletons in the room that can be resurrected by the Handy Tentacle with it's "throw juice" ability, should the fight is too easy for the PCs or they are hanging back too far. These adds are Boneknights from page 6 on this link: https://www.gmbinder.com/share/-Mv5UKsPhrHkBqSf9vo1

Conclusion and Questions

Is this a fun boss? I aim to have the tentacles each fighting a different person, so I don't think I'll wipe the party out too quickly, but I know of the mystical "action economy" and I might be overdoing things.

Also, how do I rate the CR of such a creature? And how much gold should a party earn for such a fight? Thanks for helping out a new DM!

Hello tabletop community,

I am designing my first ever boss fight, meant to really challenge my players. I don't want to kill any of them off since we're all new and it feels weird to lose a character, but I also want it to feel threatening. I could use some advice about how to execute this situation:

Boss is a big mimic disguised as a blood fountain with 6 long tentacle arms, each of which will be it's own figure on the board that can be up to 3 tiles from the 3x3 mimic figure.

Four of the tentacles have weapons that can deal 2d6 damage and have 30hp AC11 (roughly making it in line with a Brown Bear for threat and HP). They can grapple (squeeze the target) instead for 1d8 damage per turn, escapable on DC13 strength.

Two of the tentacles have shield-like hard points that defend neighboring enemies by giving disadvantage on attack rolls against them. AC14 HP30. I couldn't find anything to base these on for an encounter designer app.

Main body has low movement (10ft), and has 200hp AC19, and loses 20hp per tentacle lost. It has a bite attack +6 to hit 2d8. It also floods blood through sections of the room, forcing player movement and dealing 2d8 poison damage. I based it on a Roper for difficulty and attacks.

I think this combination creature would provide a very interesting and movement focused fight, but I'm very new to DM'ing and I'm afraid of accidentally killing the players.

My questions are: (1) How do I avoid accidentally killing everyone? (2) Is this fight too hard for a newbie group of 5 new players? (3) Any other cool ideas I didn't think of?