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Throgmorten is an ill-tempered and domineering Asheth temple cat whom Christopher steals from the temple in The Lives of Christopher Chant. The purpose of this theft was to discover if Christopher might be able to transfer a living thing across the Place Between and back into his physical world. Christopher's Uncle Ralph proposes to kill the cat and sell the pieces for a great deal of money

Understandably upset about this, Throgmorten is taken in a basket back to Christopher's home, where Christopher wakes up to find him prowling around.  Uncle Ralph's helpers have sealed the bedroom magically, but have been unable to get the cat out from under Christopher's bed, and have received a number of painful scratches in the process.

Throgmorten's unhappy noises ("Wong?" being the most frequent) and meaningful glances make Christopher feel sorry for him - sorry enough to rip a hole in the magical curtain, allowing Throgmorten to escape.

Later, Christopher encounters him again at Chrestomanci Castle, where a member of staff has brought him for his own safety.  Luckily, he remembers Christopher, seems to like him, and eventually becomes a key part of the Castle's defenses in the battle to defeat The Wraith and his evil minions. 

Throgmorten becomes the progenitor of a great many sentient and magical cats who inhabit the Castle years later in Cat Chant's time.