Had a quick glance at Alpha, I like what I see. Will need to do deeper dive when time permits.
One point I’ve sort of made before, in grittier games like Symbaroum, the heroic 5e hit-point safety net can sometimes feel inappropriate. For example In certain edge case situations like a hostage scenario, whereby a knife to the throat threatens the life of the hostage. Hostage could be a player character for instance.
In a standard game of 5e where high heroics are emphasised hit-points work well as described in the PHB , but in a grittier game I think there is room for developing a rule for edge case/conditional situations to alter the atmosphere a little.
Going back to the hostage situation, if the pc hostage has a bag of hit points, there can be a situation where the threat of a knife to the throat is meaningless as the pc knows they can survive the attack, and counter it next round.
I propose a conditional rule to maintain a sense of threat for Symbaroum, whilst not interfering with standard 5e combat and the complexity that class features can bring:
“If a target is surprised, restrained, unconscious or otherwise helpless, and suffers a critical hit, they must make an immediate Constitution Save (DC 15). On a failure the target falls to zero hit-points and death saves.”
If you wish to up the threat a little more, add something like this:
“If the damage from the Critical hit is equal or greater then their Constitution characteristic they immediately die without recourse to death saves”
I think this could work well for Symbaroum. You could alter the Con save dc to be set by Rolled weapon damage? Or have a different threshold for instant death instead of the constitution score. The scaleable half-hitpoint wound threshold in the DMG could be used if you wanted to peg such a rule to the heroic curve of 5e.
There’s precedent for this ruling in AD&D 1e as well as 3ed.