And summoning doesn't care about placement of summoned monsters, so it filled that spot with an archlich and all the other spots next to it with various high undead.
hm.. but why monsters appeared around me, not at this corner? Like when hounds breath - they do it with certain AoE range around impact point.. Summon got different mechanics?
Summon "high undead" always tries to summon 8 monsters. So first it picks the spot for the first monster, that is in LOS of the monster and at max one square away from the char (in that case just below the @'s position), then simply picks up the closest 7 spaces around that spot and fills them. In your case, there were many available spots, so all 8 high undead were brought forth...
