Unstable Reactor Core

Instability. At initiative 20, roll a d6. If roll result equals 1, the unstable reactor core becomes stunned until the next initiative 20. If roll result equals 6, the unstable reactor core instantly makes a Lightning Arc attack.

Magic Resistance. The unstable reactor core has advantage on saving throws against spells and other magical effects.

Magnetic Grasp. All creatures damaged by the unstable reactor core are pulled pulled 5 feet closer to it.

Shocking Ground. All creatures within 5 feet of the unstable reactor core take 10 (3d6) lightning damage at the start of their turn.

Lightning Arc. Ranged Spell Attack: +6 to hit, range 30 ft., one target. Hit: 13 (3d8) lightning damage, and arcs from the target to one creature of the unstable reactor core's choice 30 feet of the target. That second creature must succeed on a DC 13 Dexterity saving throw to halve 10 (3d6) lightning damage. If the second creature fails its save, the ability will arc again to another creature of the unstable reactor core's choice 30 feet of the second target. That third target must succeed on the same DC 13 Dexterity saving throw to halve 7 (3d4) lightning damage. 

Overload.  If the unstable reactor core's Lightning Arc ability misses, it can use its reaction to reroll the result. If it misses again, it becomes stunned until the end of its next turn.