Secret Lair...Amphibian God
'Amphibian God' suggests some sort of giant monster creature.
The first ideas that come to mind of how the 'lair' might look, is a damp, dark, sticky cave-like area. The egg room from Alien came to mind.
another idea perhaps is that this is a more graceful creature - so more green, warm forest like surroundings might be his home.
Here is an influence map of initial ideas for the lair.