1) Rare cards should be balanced compared to common cards
2) 5|6 stats on a Life creature are worth 5
3) If Deadly Venom is worth +3 cost when Atk>0 then Ultra Deadly Venom would be worth at least +4 cost
4) If Neurotoxin is worth +4 cost when Atk>0 and Ultra Deadly Venom>Neurotoxin, then Ultra Deadly Venom would be worth at least +5 cost
5) Why even have the 2nd ability?
6) Why/how does a Norse dragon explode? and why would there be enough venom in the body (Wyvens store poison near their fangs) to cover the entire field in a double plague?
Recommendation:
Remove 2nd ability.
Increase upgraded to 6|6.