Interesting. So this thing can only be killed by being attacked twice in one turn, depleting the full value of it's HP both times. Since it will gain 1 max hp the turn you play it, killing it on your opponent's turn will result in it having 1 hp.
Your upgraded card, though, still seems far weaker, even if it loses only 2hp instead of 4. They are still essentially the same for the first few turns only the upgraded lacks attack.
Turn 1 -> play Jellyfish
Opp 1 -> Kills Jelly fish. Jellyfish is now 4|1
Turn 2 -> Jellyfish is now 4|2
Opp 2 -> Kill jellyfish, jellyfish stays dead as it would be at a max hp of -2
Turn 1 -> play Nutricula
Opp 1 -> Kill Nutricula. Nutricula is now 2|1
Turn 2 -> Nutricula is now 2|2
Opp 2 -> Kills Nutricula . Nutricula stays dead as it has a max HP of 0
In order for Nutricula to outlive the unupgraded form it would need to survive 3 turns, otherwise killing it twice in a row will make sure it stays dead. Jellyfish will still outdamage Nutricula in those same three turns and Jellyfish needs to survive 5 turns before it can survive being killed twice.
It's an interesting concept, though, and I really like where this idea is headed but I feel it definitely needs some tweaking.
Im getting confused, i will just explain whats on the top of my head first and see replies.
Immortal Jellyfish increase max hp every turn, but does not heal 1 hp every turn.
You only need to kill Immortal Jellyfish once if its max hp is 4.
You need to kill Immortal Jellyfish twice if its max hp is 5.
Turn 1 > Immortal Jellyfish 4 attack /(4 hp out of 4 max hp)
End of turn > Immortal Jellyfish 4 attack /(4 hp out of 5 max hp)
opponent turn > lightning deal 5 damage to immortal jellyfish, Immortal Jellyfish stops death and lose 4 max hp instead, 4 attack/(0 hp out of 1 max hp),
recover to full hp > Immortal Jellyfish 4 attack/ (1 hp out of 1 max hp.)
End of turn 2 > Immortal Jellyfish 4 attack/(1 hp out of 2 max hp.)
So basically how fast Immortal Jellyfish and Turritopsis Nutricula comares to dying faster is determined by their lose 4 and 2 max hp respectively.
You will need to buff up heavily Turritopsis Nutricula in order to make it powerful yet when it "dies" it loses 2 max hp only.
Since it's stops death, does that mean death effects don't trigger like bonewall? Does that mean poison doesn't go away?
Yes death effects are not triggered until Immortal Jellyfish still dies after stopping the death effects,
poison never goes away.