As the description says, this requires you to kill Cassandra.
IN ADDITION, you must not give Arhu the Stasis Fern to become permanently cat or human.
Location: Phantom Forest, after passing through the poison log. Cassandra is located towards the North.
To kill Cassandra you must make her vulnerable (because she is a lich). To do so requires entering the Hall of the Dead in Phantom Forest. (Located on the towards the western side of the map before the swamp land).
The entrance is guarded by sentinels that spawn lava, either sneak by or use your teleportation pyramids to get to the center. You should find several dead beings in the center and a sign announcing the area, that only those who are dead can enter. Simply throw a teleportation pyramid in the center and pull the lever, this should teleport the dead beings and the pyramid inside.
Once inside follow the halls until you find a fake wall on the northside of an eastern hall that enters into Cassandra's burial site. Dig her grave up and attack her skeleton with FIRE magic. There is also a bloodstone in the massive skull's eyesocket behind the grave.
When the skeleton has been destroyed you can now kill Cassandra and remember not to talk to Arhu or give him anything.
For an extremely easy fight you can teleport Arhu out of his cage and he join in on the attack on Cassandra.
Alternatively: You can obtain the Stasis Fern and complete these achievements. (Refer to their guides for details). They are mutually exclusive and can only be obtained once per playthrough; save and reload to get all three.
Giving the Fern to Arhu unlocks:
Giving the Fern to Cassandra: