If you spawn a friendly horror from Corpathus, it may block escort pathing and won't allow the quest to complete normally. As you can't control the slime and it's friendly, it's difficult to get rid of.
I eventually got the escort to complete by tunneling around the slime.
1.3.1 Corpathus Horrors and Escorts
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Re: 1.3.1 Corpathus Horrors and Escorts
can't you use look + attack on it to kill it?