Somebody under Aventhine opened a door that does not lead anywhere in this world, and then kept standing in it until the doorway started standing in her. Meet her eyes and you have made contact with a pseudonatural creature — which, in this ruleset, is the thing that unlocks the Alienist.
The real PnP entry requirement for the Alienist prestige class is contact with a pseudonatural creature. PRC never modelled it; this module now does, and she is the contact — on sight, permanently, recorded per character.
A door tears open where the back line is standing and stays open for four rounds. No other boss in this bestiary targets the farthest enemy; every one of them takes nearest, lowest-hp or highest-caster-level.
Negative energy plus a Will save against losing the round to what you saw — and, critically, one Attunement for her. An EMPTY door pays her nothing at all, which makes the counterplay purely positional.
Permanent natural armour, attack and acid damage, and she visibly grows. Grafts never come off. Deny her the doors and she finishes the fight at her base statistics.
Aventhine Sewers' real ceiling is Sable Korrin, the Plaguecaller at CR10; Ilqaeth is pitched one notch above at CR11. The area's trash tops out at the CR5 Sewer Troll, so the boss tier is the whole difficulty of the place — she sits at the top of it without running away from it.
She is the bestiary's first Alienist and the only boss anywhere in the module that changes a player character's sheet. Wizard 5 / Alienist 6 is caster level 11, which buys real 6th-level wizard spells (Acid Fog, Greater Dispelling) rather than a bolted-on spell-like list; Evard's Black Tentacles on the 4th-level line is doing thematic work as well as mechanical.
No epic spell, and honestly so: Epic Spellcasting needs caster level 21, and this area caps at CR11. Faking one here would break the CR match the brief asks for. Same call as Halvard, Nyssara, Amareth, Kessa and Coralynn last round.