Psionic Resilience. Volgrun has advantage on saving throws against being charmed or frightened, and on Constitution checks to maintain concentration.

Stone Song. Moving along natural stone doesn’t provoke opportunity attacks from Volgrun. Difficult terrain of natural stone costs him no extra movement.

Innate Psionics (Save DC 16; +8 to hit). At will: detect thoughts, levitate (self only). 3/day each: shatter, suggestion, hold person. 1/day: telekinesis.

Psionic Greatmaul. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. 15 2d8 + 6 bludgeoning plus 7 (2d6) psychic damage. If the target is a creature on natural stone, it must succeed on a DC 16 Strength saving throw or be knocked prone as the ground ripples.

Tentacles. Melee Weapon Attack: +10 to hit, reach 10 ft., one creature. 17 (2d10 + 6) psychic damage, and the target must succeed on a DC 16 Intelligence saving throw or be grappled (escape DC 16), stunned until the grapple ends. While grappled this way, the target takes 10 (3d6) psychic damage at the start of each of its turns.

Synaptic Siphon (Recharge 6). Volgrun targets one creature grappled and stunned by his Tentacles. The target takes 21 (6d6) psychic damage. If this damage reduces the target to 0 hit points, the creature is stable but gains one level of Exhaustion and −2 to Intelligence (restored by greater restoration or a long rest plus DC 15 Medicine care). On a success against the Tentacles save this turn, this action fails.

Mind Blast (Recharge 5–6). Volgrun emits psionic energy in a 60-foot cone. Each creature in the area must make a DC 16 Intelligence saving throw, taking 22 (5d8) psychic damage on a failed save and being stunned for 1 minute. A creature can repeat the save at the end of each of its turns, ending the effect on itself on a success.

Stone Ward (Recharge 5–6). When Volgrun is hit by an attack, he hardens the stone around him, gaining resistance to that attack’s damage and to nonmagical bludgeoning, piercing, and slashing until the start of his next turn.

Volgrun can take 2 legendary actions, choosing from the options below, at the end of other creatures’ turns.

Seismic Step. Volgrun moves up to 20 ft. along natural stone without provoking opportunity attacks. Creatures that start their next turn in the 10-ft-wide path must succeed on a DC 14 Dexterity saving throw or fall prone as the stone slithers.

Tremor Pulse. Each creature on the ground within 10 ft. must make a DC 14 Dexterity saving throw or be knocked prone.

Tentacle (Costs 2). Volgrun makes one Tentacles attack.