A minor death bless (D4-8) increases the chance of your blessed sacred units (or blessed sacred mages) causing an infliction on those they damage.
I think some people like to do this on archers. LA TC has sacred Ancestor Vessel horse archers, but I can't remember if their howling bow messes up some bless effects..?
EA Kailasa has sacred archers too, so they could use a death bless to increase inflictions on the victims of their archery. I don't think too many people do that though - the monkeys are pretty vulnerable to archery, so picking an archery fight with people tends to not be something Kailasa wants to do (but maybe someone has made this work?). A not terribly good idea would be your archers raining affliction causing shafts down on your archery vulnerable melee troops as they engage the enemy. The screeching you hear is not monkey happiness.
Having blessed thugs that are more likely to cause afflictions can sometimes scare your opponents. Other players may not want to send a well kitted-out thug or SC against your thugs with their increased affliction chance.
Sacred mages raining evocations down on the enemy get the bonus affliction chance too. Get hit by a fireball and lose an eye! Or use the blood large AOE low damage spells (ie. Agony, Harm, Bloodletting) to give afflictions to an entire army.
Major death bless (D9+) provides a bonus AN damage (MR save negates) to melee attacks.
The major bless (like the major fire bless) interferes with the secondary effects on popular forged weapons used by thugs/SCs, like the frost/fire/death brands. When you want AOE fire for the entire square you're attacking and all those gems you spent forging that rocking gear only get you some +AP fire damage to the single target you hit, this is frustrating.
Of course, you can always ignore the bless effects and just decide that it's useful to have a pretender with access to high death magic for it's own sake. If you wanted a nice bless, why didn't you take water or nature or earth?
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Edit: Too slow. Ninja'ed!