untested, might work anyway lol

monkestation/code/modules/antagonists/vampire/actions/tenacity/fortitude.dm

@@ -11,17 +11,58 @@
 	name = "Fortitude"
 	desc = "Enhance your body even further at the cost of speed. Drains lifeforce while active."
 	button_icon_state = "power_fortitude"
+	cooldown_time = 1 MINUTE
 	toggleable = TRUE
 	constant_life_cost = LIFEFORCE_PER_HUMAN / 300 // 5 minutes of fortitude per human.
 
+	/// How much damage Fortitude can still resist before breaking and causing backlash.
+	var/durability = 100
+
 /datum/action/cooldown/vampire/fortitude/on_toggle_on()
+	durability = initial(durability) * (vampire.get_stat_modified(VAMPIRE_STAT_TENACITY) / VAMPIRE_SP_MAXIMUM * 2) // Fortitude is granted at half points to max, meaning the scaling starts at 1, then reaches 2 at max.
+
 	user.set_armor(user.get_armor().add_other_armor(/datum/armor/vampire_fortitude))
 	user.add_movespeed_modifier(/datum/movespeed_modifier/vampire_fortitude)
 
+	RegisterSignal(user, COMSIG_MOB_APPLY_DAMAGE, PROC_REF(on_apply_damage))
+
 	to_chat(user, span_notice("You enhance your body past it's limits. No one may hurt you any longer."))
 
 /datum/action/cooldown/vampire/fortitude/on_toggle_off()
 	user.set_armor(user.get_armor().subtract_other_armor(/datum/armor/vampire_fortitude))
 	user.remove_movespeed_modifier(/datum/movespeed_modifier/vampire_fortitude)
 
+	UnregisterSignal(user, COMSIG_MOB_APPLY_DAMAGE)
+
 	to_chat(user, span_notice("You stop enhancing your body, freed of the burden once more."))
+
+/datum/action/cooldown/vampire/fortitude/proc/on_apply_damage(datum/source, damage, damagetype, def_zone, blocked, wound_bonus, bare_wound_bonus, sharpness, attack_direction)
+	SIGNAL_HANDLER
+
+	if(damagetype != BRUTE && damagetype != BURN) // Don't bother with non-lethal damage types or suff/tox and whatever else.
+		return
+
+	durability -= damage
+
+	user.visible_message(
+		message = span_danger("[user]'s body flashes an iron grey!"),
+		blind_message = span_hear("You hear a clang!")
+	)
+
+	if(durability <= 0)
+		shatter(attack_direction)
+		return
+
+/datum/action/cooldown/vampire/fortitude/proc/shatter(attack_direction)
+	user.visible_message(
+		message = span_danger("[user] reels back in anguish!"),
+		self_message = span_userdanger("Your Fortitude shatters!"),
+		blind_message = span_hear("You hear something shatter!")
+	)
+
+	if(attack_direction)
+		user.Move(get_step(user, attack_direction))
+
+	user.stamina.adjust(-STAMINA_MAX * 0.25) // Puts you above the stun threshold, but below the exhaustion threshold, thus inflicting the latter.
+
+	toggle_off()