The Thunder Spirit (in-game name Sähkiö) is a flying electric skull enemy, very similar to Jäätiö. It attacks with a short-range electrical strike similar to those cast by Ball Lightning, and has an electrical aura that sends dangerous currents through any nearby conductive surface.
Thunder Spirits fly around slowly, pursuing the player and attempting to get close enough to fire its short-ranged primary attack. As they move about, any nearby material that happens to be conductive will be lit up with electricity similar to how the Electricity perk functions, though with a wider radius.
Close-range combat is not recommended as their electrical short-range attack will stun the player for a few seconds, and may prove fatal if the player happens to fall onto a conductive surface too close to a Thunder Skull's electric aura. It is especially dangerous in The Vault, where large sections of the level have metal surfaces, and its electrical aura can easily be spread throughout the metal over quite a distance.
Prefer long-range attacks and avoid standing on metal surfaces that may become electrified. Electrical current from a surface near a Thunder Skull may easily travel through any connected metal objects or terrain, so be cautious about standing on or touching anything metallic while they are around.