I think it's better to compare this with parasite, the ability is sorta similar and the duo needed to use is the same.
This ability is less susceptible to purify compared to infect, but if your opponent doesn't have purify this deals less damage to creatures in the long run. So I think you can safely lower the ability cost to
. About the summoning cost I think you should try to estimate how much invisibility is worth and add this to parasite's cost. Also, take into account the difference in attack in the cost.
About the art ... I see what you did there.