Obsidian Ophidian

Lava Walker. Can move across lava surface as if harmless solid ground. In addition can swim through lava at its walk speed.

One with Lava. Can't be surprised by creatures within 5 ft. of lava and can sense any creature touching same pool of lava it is touching.

Sharp Fragments. When it takes bludgeoning each creature within 5 ft. of it takes 2 (1d4) piercing damage as shards of obsidian fly from it.

Volcanic Rejuvenation. Regains 10 hp at the start of its turn if at least part of its body is in contact with lava. If it takes cold dmage this doesn't function at the start of its next turn. Dies only if it starts its turn with 0 hp and doesn't regenerate.

Multiattack. Two Biteks or one Bite and one Constrict.

Bite. Melee Weapon Attack: +6 to hit, 10 ft., one target, 12 (2d8+3) piercing damage. Target: DC 15 Con save or take 5 (2d4) piercing damage at start of each of its turns as shards of obsidian lodge in wound. Any creature can use action to remove shards with DC 12 Wis (Medicine) check. Shards crumble to dust if target receives magical healing.

Constrict. Melee Weapon Attack: +6 to hit, 5 ft., one target, 8 (2d4+3) bludgeoning damage and target is grappled (escape DC 15) if it is a Large or smaller creature. Until this grapple ends target is restrained and takes 5 (2d4) slashing damage at the start of each of its turns and ophidian can't use Constrict on another target.

Lava Splash (Recharge 4-6). Writhes and flails splashing lava at everything. Each creature within 20' of it: 21 (6d6) fire (DC 15 Dex half). To use this must be touching or within 5 ft. of lava.