I like that he has a different progression path. That's interesting.
However, it needs more balancing. Particularly the stat conversion. While I get he's supposed to be a close range mage, he still has a skill that is affectively "I Win" give or take a randomizer. So I would say that the Stat Converter needs to be an extension of Death Coffin.
Theorhetical Name: Memento Mori
Description: All Targets currently gathered within Death Coffin will have a portion of their stats accomodated to the User. This last until they are removed.
Additional Edit To Death Coffin: Consumes additional mana when Memento Mori is being used. Targets are expelled when User runs out of Mana.
In this way, both skills aren't just Convenient Lvl Up Stats and Convenient Inventory Box like they tend to be in every cookie cutter story like this one. No, now they are intergral parts to how an Instant Death Mage plays. You don't just get to have a "Win The Fight" Skill along with "Level Up Like Everyone Else" Skill. The point in being Capped at Lvl 1 is to balance your job that has a progression path which is very unique.
If you just repeat what everyone else does on top of "I Win The Fight" Skill, than that's just ludicrous. If I had to take it a step further, the main Skill would have to require the target (and perhaps even the User) be at a certain percentage of health (like Executions from SMITE).