Arcing Spark fires now and then when you hit something. On Faded Torrent Relic it reads: Hits have a 20% chance to arc to a nearby enemy for 12% of the damage.

Word for word it is Hits have a {chance}% chance to arc to a nearby enemy for {value}% of the damage, and the number in it grows with the gear: 35 to start, 2 more for every item level, at a 20% chance.

9 bases carry it, and 3 Uniques are built on those: Undertow, The Glassblower's Breath and Thunderroc's Halo.

Awakening can reach it too — 22 spirit paths pick it up at Spirit Level 3: Cinderveil, Rimebolt, Voltweft, Venombolt, Sleetcall and Toxincall and 16 more.