Using the hit die as a soak damage mechanic in Adventure TTRPG

Two of my goal designs for Lunchtime Dungeons are: simple procedures for shorter sessions and deadly combat. With that in mind, I'm going to test the following hack. I'm open to thoughts and suggestions.

Pathetic aesthetic by Jim Holloway

1. Damage is a fixed number (the max damage of the weapon). A modified 20 on the attack roll doubles it. Monsters (and maybe fighters?) add their HD to that. So a goblin with a club would do 7 damage, and a vampire with a greatsword would do 17.
2. Characters have 2d6 Stamina* and one hit die**, ranging from d8 (fighters) to nothing (wizards). They get to subtract their HD from each blow, and the remaining damage is deducted from their Stamina. 0 Stamina means a roll on the death & dismemberment table.
3. Monsters lose hit points as normal. I'm going to use d10s for hit dice so that some goblins can withstand a blow from a long sword.

 * You can use Constitution in your six-stats game.
** I'll probably rename it to Soak Die or something.