OK, this is not exact. It may be incorrect. I searched in different sources to contrast the results and every one gave the same result. But again, this is Ancient Greek so…I did my best:

First of all, the word “katerkheo” doesn’t exist. Or at least none of the dictionaries and other sources where I’ve been looking recognize it. This particular word took me most of the time, actually, find the word that matches the phonetics. I think is Katakurieu, and with the meaning it’s pretty clear for me:

Drakon: Dragon. (^^ I broke my mind on this one XD)
Fthengomai: It means emitting a sound or a voice.
Kalon:  summoner.
Su: you.
Katakurieu: be lord over, exercise dominion over, exercise lordship over, overcome, lord it over, lording it over, subdued. (Bingo, don’t you think?)
Deuro: hither to this place.

So, pretty much what (again, this is my thing and it’s not an official translation) Merlin says is:

O Drakon, fthengomai au se kalon su katakurieu deuro!

Oh Dragon, my voice summons you and it forces to come here!

I hope it helps you, I did my best ;)

EDIT: This is by far the hardest question nobody asked me here. Please, tell me who you are! And thank you for the challenge! It was funny XD