Paladins are the more battle orientated of the two classes and get use of the Holy Avenger spell. This is fine. I was thinking however that since paladins can use turn like a cleric but not with the same strength, a cleric should be able to get their version of Holy Avenger but with a little less bonus. At this point, it might seem fair, i think so.
But just in case the argument isnt good enough i would suggest allowing the cleric holy avenger spell to ONLY work on maces and hammers. This means clerics cannot sever the undead but would gain a bonus against them and be able to fracture. Making sense since i think all bludgeon weapons should fracture the undead (not sure if they actually do so atm
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I don't know how many tri-avatars there are with a base class of Cleric, but mine was first.