Summoning of Arachnee

Acquisition
Summoning of Arachnee is learned from Aria Knofob at (11,8) after you help her clear out the spiders in the attic. The spiders are actually two level 2 arachnids.

The spell is learned directly from the NPC, and the proof of victory (a Leg of Arachnid) is a quest item, so the only way to acquire it is by fighting the spiders yourself. Only one player can fight on the attic map, and the mob has a fairly long respawn time (believed to be around 6-8 hours) so expect some waiting.

For people unfamiliar with arachnids, they can summon other arachnids as well as arachnees. One spider can summon another, which summons another, which summons another... The end result is up to 8 new spiders summoned per turn, but if you kill either of the original arachnids, everything it summoned dies as well.

Non-LoS and area spells are good for killing originals even when surrounded. Teleport spells let you enter melee before they can block you with summons. 10+ physical damage reduction (e.g. Warrior Fecalizer) makes you practically invincible even without healing.