This weird creature looks like a giant tumbleweed whose form crackles and sparks with electricity.
Tumblespark CR 4
AC 19, touch 14, flat-footed 15 (+3 Dex, +1 dodge, +5 natural)
hp 37 (5d10+10)
Fort +6, Ref +7, Will +1
Defensive Abilities insubstantial form; Immune electricity, elemental traits
Weaknesses vulnerability to water
Speed 30 ft., fly 30 ft. (perfect)
Melee slam +8 (1d6+3)
Special Attacks arc, trample (1d6+3 plus 1d6 electricity, DC 14)
Individual tumblesparks are drawn to positively charged beings such as those wearing large amounts of metal armor. They slam into them discharging blast after blast of electrical energy until the positively charged victim loses its own charge usually as a result of the victim’s demise. Against smaller foes, a tumblespark simply rolls over them, dealing crushing and electricity damage.
Once per round, a tumblespark can release a jolt of electricity against a single opponent within 10 feet that deals 4d6 points of electricity damage. A successful DC 14 Fortitude save reduces the damage by half. The save DC is Constitution-based.
A tumblespark’s natural form is a ball of dancing and arcing electricity; it is not solid, nor is it incorporeal. Any piercing or slashing attack has a 20% miss chance against a tumblespark. Bludgeoning attacks deal damage normally.
Environment Quasi-Elemental Plane of Lightning
Organization solitary, pair, or bramble (3-6)
Riding the strong winds and electrical storms of the Quasi-Elemental Plane of Lightning, tumblesparks seek constantly to unleash their pent up electric energy. A tumblespark appears as a man-sized sphere of electricity, rolling and sliding across the ground. Its form crackles and dances with bluish-white electricity. A tumblespark can grow to a diameter of 10 feet or more. As they spin and move they continually generate huge amounts of electricity. Magnetism draws them towards living creatures until the spark is fully discharged. Unfortunately for planar travelers unlucky enough to encounter a bramble of these creatures, this energy never seems to fully dissipate.
Tumblesparks are sometimes summoned to the Material Plane by spellcasters to safeguard some precious item or location, but more often than not any encounter with a tumblespark on a material plane is the result of the creature rolling or crashing through a portal. On a material plane, a tumblespark is likely to be encountered in areas of great electrical activity.
Tumblespark from the Tome of Horrors Complete, Copyright 2011, Necromancer Games, Inc., published and distributed by Frog God Games; Author Scott Greene.