(Next time, present the challenge to me, rather than simply posting it.)
Because Valor was attempting to use a spell to forcibly bring a mage, and a powerful one at that, to him, Nisorin was easily able to detect it. Teleporting in before Valor's spell was detected, he smirks and promptly uses a fireball to disrupt the runes designating the summoning's destination, forcing the spell to fail. "You really didn't think I'd let you force us here, did you Starclanner?"