Pile of Bones - Undead Creature - could use some feedback please

[Rogozhin]

This is one of my first homebrew creatures as a DM. It is a pile of bones, being picked at by some hungry ravens. Animated by some dark necromantic magic it lurches to life and attacks. The party of 5 lvl 4 PCs are going to encounter this in a small prison courtyard. There may also be some skeleton archers firing from the ramparts.

Any feedback would be greatly appreciated

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Pile of Bones
Large undead, neutral evil

Armor Class 14
Hit Points 85 (10d10 +30)
Speed 15ft.

|STR|DEX|CON|INT|WIS|CHA|

| 18 | 8 | 16 | 5 | 10 | 5 |

Damage Vulnerabilities Bludgeoning
Damage Resistances Piercing and Slashing damage from non-magical weapons that aren't silvered
Damage Innmunities blinded, deafened, exhaustion, poisoned
Condition Immunities blinded, deafened, exhaustion, poisoned
Senses blindsight 60 ft. (blind beyond this radius), passive Perception 10
Languages understands commmon but can't speak it
Challenge 4 (1100 XP)

False Appearance. While the pile of bones remains motionless, it is indistinguishable from a normal pile of bones.

Skeletal Resilience Any time the pile of bones takes 10 non-bludgeoning damage in a round of combat the fallen bones reform to become a skeleton with 10 hp. It has an unarmed claw attack (+3 to hit, 1d6 slashing damage)


Actions
Multiattack The pile of bones makes two bone whip attacks.

Bone Whip Melee Weapon Attack: +5 to hit, reach 10ft., one target. Hit 8 (1d8 + 4) The target is grappled on a hit if it is a creature of medium size or smaller (Escape DC 14). The pile can use it's squeeze action on a grappled creature instead of one of it's bone whip attacks.

Bone Squeeze The Bone Pile squeezes a grappled creature causing 1d6+4 piercing damage.

[Cap'm Bubbles]

Damage Immunities currently match conditions.

The choice of vulnerable-bludgeoning while resistant to the rest of physical damage is understandable, but this has a tendency to annoy players that picked different weapons for flavor rather than function (very little in the Monster Manual takes the approach you have here). Sure, it's fun to throw a curve ball at that half orc barbarian with his great ax, but if you don't know ahead of time that your party will have a source of bludgeoning damage (not as common as you might think), this fight could get very frustrating.

Any caster with Ray of Frost can shut down this fight handily (this pile isn't going anywhere at 5ft movement whenever that hits). In general this fight would favor casters over melee fighters significantly.

The wording on skeletal resilience is unclear. Is this 10 damage per round able to be cause multiple times in the same round (i.e. a caster causes 20 damage with a Chromatic Orb spell; is that 2 skeletons or 1)?

When accounting for skeletal resilience, this creature would be better described as a CR 5, or CR 6 for a party without bludgeoning damage.

Overall, this enemy can be intimidating and a challenge to a low level party. It would work best either surprising a character investigating/looting the pile, or actively guarding a small-ish room while enemies outside the room put pressure on the party to prioritize targets. Pretty good for your first homebrew, the flavor of it is enjoyable and its not so campaign-specific that it's good for use in a lot of settings.