The Oblivion Cleric's Channel Divinity "Mark of Fate" is supposed to add 5 extra necrotic damage per attack received by a target. In theory it should either work one of two ways with Magic Missile;
1- If a marked creature is hit by a missile they should take 5 bonus necrotic damage, and if any more missiles hit the creature they don't take any more bonus necrotic damage
Or
2 - The marked creature takes 5 bonus necrotic damage per missile that hits them (this is overpowered as hell by the way!)
What is ACTUALLY happening is insane - the marked target is taking bonus necrotic damage once for each time they are TARGETED by magic missile, and it is then REPEATING after each missile hits them.
Just so you understand what I mean here is a copy pasted log from a combat I just did; I targeted a Fire Elemental with Mark of Fate and then aimed a 5th level Magic Missile at it while it was on full HP.
>Fire Elemental sustains damage 5 Force
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 3 Force
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 5 Necrotic
>Fire Elemental sustains damage 2 Force
>Fire Elemental sustains damage 5 Necrotic ... ETC
Needless to say this one cast of magic missile + this mark killed it from full health. With a Shock Arcanist and Arcane Fury to add yet another bit of damage it gets even crazier. AND there's another layer to this bug on top of all of this. Once you have done this once, until you restart your game for some reason every single time you cast Magic Missile on ANY enemy with ANY of your characters the target will all get showered in the same necrotic damage in just the same way and die instantly even if they aren't under the effects of the Mark of Fate. Absolutely bonkers.