I came up with more flavourful and adjustable idea for poison.
Make "poisoned" status that has counter on its tag that indicates after how many rounds unit will die from it.
"Poison" ability would have number that would decrease "poisoned" status counter of targeted unit. (from base number 10 if unit had no "poisoned" tag)
For example: Unit with ability "poison 3" uses it on unit with no "poisoned" status. Targeted unit has now "poisoned" with 7 on its counter (10-3 as 10 is base number).
Next turn unit with "poison 6" uses it on the same unit which now has 6 on its "poisoned" counter (as 1 turn passed), so after another poisoning its counter reaches zero (6-6) and poisoned unit dies.
Make "poisoned" status that has counter on its tag that indicates after how many rounds unit will die from it.
"Poison" ability would have number that would decrease "poisoned" status counter of targeted unit. (from base number 10 if unit had no "poisoned" tag)
For example: Unit with ability "poison 3" uses it on unit with no "poisoned" status. Targeted unit has now "poisoned" with 7 on its counter (10-3 as 10 is base number).
Next turn unit with "poison 6" uses it on the same unit which now has 6 on its "poisoned" counter (as 1 turn passed), so after another poisoning its counter reaches zero (6-6) and poisoned unit dies.