2 words. Over. Powered.
You do realize that Royal Swamp Eel is a Fish, right? You also hopefully realize that the other card that has an effect like this is Coelacanth, who is 8 stars and has to discard a card to use this effect? Also, Coelacanth's effect makes it so those monsters have no effect and cannot attack...
So let's see, I summon 2-3 Royal Swamp Eels and some other 4-star Fishies, synchro for 2 8-star monsters, normal summon another monster, sync for a 3rd 8-star monster, and then beat my opponent with the 3 Synchro monsters this card has provided me? Not to mention none of my monsters are going to be removed from play since they've made it safely back in the grave so I'm also not going to be taking any damage. Yes, that's brilliant right there. Most OP'd card I've seen in a very long time.
0/10.