Situation. PC is stuck to spitting mimic and is having face chewed of. (Hit the pc with points to spare on the bite.)  Rune knight uses Cloud Ruin on mimic to force the attack to another critter.  No one is within 15 feet of mimic and  only 2 other critters are in 30 feet. I ruled for the session, the attack on the pc stood due nothing being in bite range. Yes good ruling. OR Bad DM no cookie. 

The wording of Cloud Rune says that the attack can be transferred regardless of the range of the original attack, from a creature in 30ft of you to a different creature within 30ft of you, other than the attacker. As such, once you decided to do it, then the attack has to hit anybody except the original target - if no other creature is in range (it's just the rune knight, the mimic and the original target) then the effect has to put the damage on the Rune Knight, as the only eligible alternative target.

With the rule explicitly stating that it disregards the attack range when redirecting the damage, the power should have worked - I'd recommend letting thr group know you got it wrong this time and have since worked out the right way to run it.

Thanks. Visually Some of the rune knight abilities are just freaking this old fart out.