Priestess

Priestess character. A tank, 'cause a) that's the mechanic I came up with and b) priestesses can be tanks too!

So basically, her core mechanic is her unit ability (that one you can't level) that is basically "Prayer". She channels for a second or two to refresh her miracle counter. She can only use abilities ('cept her ult) if she has a miracle available and they cost miracles to cast, how many you get per prayer TBC. Basically, you want to attack her while she's praying so she can't be disruptive - typical sort of tank I guess. Her miracles (i.e. base abilities) could be blinding enemies with holy light, removing/reducing CC on allies, healing (don't particularly want to stoop to having a healer, I'm thinking she's more like Janna than Soraka), knockback, maybe teleportation or something.

Ultimate - this is why you want her to die first. On an ally dying, she can channel for a little bit, and the champion who got the kill is struck with a form of divine retribution, maybe damage, preferably some sort of strong CC, something game changing that won't be an instant kill. Thinking it could even be global, allowing for some interesting shenanigans where you try to force 3v4 fights that she turns up late to. Maybe that'd be fun, maybe it's a horrid idea.