Not sure if this was mentioned on the thread already, "returns to your hand if you do not already hold 8 cards." isn't needed since you have to discard a card if you have 8 cards before creatures attack. I also agree with the fact that theme is somewhat off. A swallow sounds like a strength 1-4 creature to me. 8 ATK seems like it's pushing it a bit.
Regarding balance : See Twin-Headed Wyvern | Twin-Headed Wyvern (
http://elementscommunity.org/forum/index.php/topic,20663.0.html).
Comparing the upped (most similar versions), Twin-Headed Wyvern pays
to stay alive each turn, while Swallow pays
. Both do 8 DoT if upkeep is payed, ignoring shields or healing.
The difference is that multiple Swallows can prevent each other from going back to the hand (1-8 swallows fly back to max out the hand size, the others just stay there. Wyvern dies if you don't pay, period. It sounds fair, but I have one question : is the ability active or passive? I'm assuming it's passive, but you could make some powerful combos with ability negation like Mitosis and Lobo if it was active.