Bright Plasma

Ticking Bomb. Every time Bright Plasma takes damage from an attack or spell effect, it grows in size, up to Large.

Unstable Elemental. If the Bright Plasma ends its turn while Large, it automatically uses it's Bomb (Large) action, even if there's no targets for it to touch.

Bomb. Bright Plasma touches a creature and explodes, self-destructing as part of this action. This action has increased effects depending on [monster name]'s size when it happens.

Bomb (Small). The Bright Plasma touches a target and self-destructs. The target must make a DC 12 Constitution saving throw. It takes 2d8 [thunder] damage on a failed save, and is pushed 10 feet away straight from where Bright Plasma was. On a success, the target takes half damage and isn't pushed back.

Bomb (Medium). The Bright Plasma touches a target and self-destructs. All creatures in a 15-foot radius must make a DC 13 Constitution saving throw. Each creatures takes 3d8 [thunder] damage on a failed save, and is pushed 15 feet away straight from where Bright Plasma was. On a success, a creature takes half damage and isn't pushed back.

Bomb (Large). The Bright Plasma touches a target and self-destructs. All creatures in a 25-foot radius must make a DC 14 Constitution saving throw. Each creatures takes 4d8 [thunder] damage on a failed save, and is pushed 20 feet away straight from where Bright Plasma was. On a success, a creature takes half damage and isn't pushed back.