Bone Titan

Skeletal Swarm. When the bone titan's hit points are reduced to 80 or less, its size becomes large.

Absorption. Any willing skeleton that comes within 10 feet of the bone titan can be absorbed into it. It gains the hit points the skeleton had. If the bone titan exceeds 500 hits points, its size becomes gargantuan, and it has advantage on all saving throws.

Risen. At the beginning of the bone titan’s turn, any corpse within 120 feet of the bone titan is resurrected as a skeleton. If the corpse is not skeletal, the skeleton is ripped out from it. If the corpse has no skeleton, this ability has no effect.

Split. As often as the titan likes, at the begininning of its turn (after its risen ability), it can choose to lose 13 hit points and produce 1 skeleton. 

Shed. For every 10 damage the bone titan takes, roll 1d4. On a 4, the damage is reduced to 0 a skeleton is flung from the titan.

Perceptive. The bone titan adds its proficiency bonus (+5) to its perception. It also has advantage on any perception checks that rely on sight.

Multiattack. The bone titan makes three sword claws attacks. It can swap an attack for a slam or coldflame breath (if available). If the bone titan's size is large, it can only make two sword claw attacks (one of which can be slam or coldflame breath)

Sword Claws. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 14 (3d6 + 6) slashing damage.

Slam. The bone titan slams the ground in front of it, generating a shockwave in a 10-foot cone. Any creature in the area must make a DC 19 Dexterity saving throw or take 10d10 bludgeoning damage. The bone titan loses 40 hit points and two skeletons are split from the bone titan and flung in a random direction for 2d20 feet.

If the titan's size is large, this attack deals half damage and does not spawn skeletons, nor does the bone titan take damage for using the attack.

Coldflame Breath (Recharge 6). The titan exhales blue coldflame in a 30-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 35 (10d6) cold damage on a failed save and half as much on a successful one.

The titan can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The titan regains spent legendary actions at the start of its turn.

Bows (costs 2 actions). For every 40 hit points the bone titan has, it makes a bow action.

• Bow. Ranged Weapon Attack: +4 to hit, range 80/320 ft., one target. Hit: 5 (1d6 + 2) piercing damage.
   

Necromantic Implosion (costs 1 action). The bone titan selects 1d4 skeletons within 120 feet that it can see. Those skeletons explode, dealing 10 (3d6) necrotic damage to any creature within 15 feet of the explosion. The skeleton also takes equivalent damage. If it is still alive, it reforms at the beginning of its turn.

Skeleton Fling (costs 2 actions). The bone titan loses 13 hit points and flings a skeleton for up to 60 feet. The skeleton can make a sword attack against up to two creatures within 5 feet of it during travel. Additionally, if the skeleton collides with a target, both the skeleton and the target take 10 (2d6 +3) bludgeoning damage.

The titan can choose to instead, expend three legendary actions instead, detonating the skeleton at will, dealing 11 (3d6) force damage and 11 (3d6) necrotic damage to any target within 15 feet of the skeleton.

Sword Claws (costs 1 action). The titan makes a sword claws attack.