Yggdrasti

The yggdrasti has 1d4 + 2 cavities in its trunk. Each cavity is big enough to hold one Medium creature, two Small creatures, or eight Tiny creatures. A creature inside a cavity has Cover against attacks and other effects that originate outside the cavity. The yggdrasti's cavities aren't connected to one another.

If the yggdrasti is motionless and rooted in the ground at the start of combat, it looks just like a dead tree and has advantage on its initiative roll. Moreover, if a creature hasn't observed the rooted yggdrasti move or act, that creature must succeed on a DC 18 Intelligence (Investigation) check to discern that the yggdrasti is animate.

If the yggdrasti is subjected to Lightning damage, it is unhurt, and the Lightning damage is instead divided evenly among all creatures it is grappling. In addition, the yggdrasti regains one use of Lightning Discharge.

The yggdrasti doesn't require air or sleep.

The yggdrasti makes two Root attacks and uses Lightning Discharge (if available).

Melee Weapon Attack +8 to hit, reach 20 ft., one target. 12 (2d6 + 5) Bludgeoning damage, and if the target is a creature, it is Grappled (escape DC 15). The yggdrasti has four roots, each of which can grapple one target.

The yggdrasti shoots lightning at one creature within 120 feet of itself. The target must make a DC 13 Dexterity saving throw, taking 31 (7d8) Lightning damage on a failed save, or half as much damage on a successful one.