The sewers changed Meridax before the ooze ever finished changing him back. Drop him to half his blood and there are, briefly, two problems where there used to be one.
A second avs7_meridax spawns beside him with the current HP split evenly between both bodies. No prior boss in this bestiary duplicates its own body mid-fight - Mother Bellow's Cave-Totem (round 9) is a separate support creature, never a copy of herself.
His epic spell answers on both halves at once - reflavored the Rot Ascendant, a temporary hardening of natural armor, acid damage, and slashing resistance - rather than a scheduled autocast.
The partner body dies instantly too. There is no second full fight hiding behind the split - it changes HOW the fight plays, not how long it lasts.
Aventhine Sewers' real ceiling was Vharok Deepmark at CR13 (round 9); Meridax is pitched one notch above at CR14, into the area's last remaining Sparky slot (enc10).
He is the bestiary's first Oozemaster - an ability-based PRC (no dedicated spells.2da column) built specifically around the class's own fluff, cloned from the module's bele_seahag donor for its amphibious, already off-human appearance.
Real Epic Spell: Dragon Knight (id 638/875) - Aventhine Sewers already had Ruin (Vaerith Colm) and Mage Armor (Vharok Deepmark); Dragon Knight went here instead, its brief-transformation flavor matching a body that's already splitting into something else.