Your question is a little too vague, I think, but here are a couple I can think of:
There is a high-level locked door in one of the Liche's rooms. This can only be opened by a super-advanced thief, a really strong character, or using a key gained from an optional side-quest.
There is a locked door with a face. This doesn't need a key, it needs an item gained from the Liche.
The Liche's door is opened with a key found in the dungeons.
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