How about this?
*Group Shattering Strike
100% resistance to Stun
Devouring Impact: Target lowest HP. Destroy shields. Deal 1.5x damage bypassing armour. Regenerate 1x. Delay:0, Cooldown:2. (Note the lack of Cleanse)
Total Annihilation: Target all. Destroy shields. Deal 1x target’s Max HP, bypassing armour. Delay:0, Cooldown:3 (Note that this also targets itself, and every creature on the field)
Cunning creatures will still wreck it, especially ones that have actual Cunning moves.
Total Annihilation follows the classic “too much power to be controlled” trope, and while it KO’s Mortem (unless you distract it), it would allow an unboosted Mortem Rex to OHKO any resilient creature, regardless of boosts, assuming it’s set up. That isn’t to say that it would be infallible to higher-level resilient creatures, but it would definitely be close.
It would be the JWA equivalent of a nuke, especially in raids, which I think is fit for the “Death king”.
It can be used in raids if the opponent distracts Mortem (the irony) or if the teammates are sufficiently rend-resistant.
Being the slowest creature in the game, it can be revenge-killed by anything.