I think it's possible that Ilberd's intent was to summon Rhalgr (compare the early-game plot in Little Ala Mhigo where the Ascians are encouraging them to do the same) but somewhere along the line it got mixed with the concept of a dragon - whether because of the dragon's eyes he was using as a power source, or just looking for a more imposing form - or perhaps because his original plan was interrupted by us, he didn't have as firm a grip on exactly what he was summoning.

I also think it's likely he wasn't going to sacrifice himself, but only did so because (due to our unplanned victory over him) he was dying anyway. Thus, he wasn't around to control the summoning as it took form. If we hadn't intervened, perhaps it would have been Rhalgr that manifested.

(Running from memory on all this, though. I haven't rewatched those cutscenes since I first saw them.)