I believe it's suppose to give the caster Darkfire, but currently it does nothing.
My PC is Vortigern Clale, my Domains are Magic/Metal- that's how I've tested it.
It's under assigned special abilities-Cleric.
I've been playing 3 weeks- it hasn't worked that entire time, the reason I never reported it before was due to me originally assuming it was perhaps due to not having the correct Relic.
set secondary domain power to a player tool and use it through the player tool