I've never been a big fan of no-save death attacks.
A dread wraith can kill a character in 2-3 hits, easy. DC 25 fort save isn't even a sure thing for most players at the relevant CR.
At a +16 touch attack and having incorporeal traits, I think they are already a signifigant challenge for their CR. They will never miss, and have probably a 50/50 shot at con draining a fighter type...more like an 80% chance to drain a caster or rogue type.
I'd say +2 CR off the top of my head if you wanted to make the drain no-save....But then again as a player, if I knew they didnt allow a save then I would run from them. Like 1e/2e undead that drained levels....I'd rather die and make a new character than have to re-do 2 or 3 levels.