I know it is a silly Question but I have forgot where you get it
Thanks.

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I take it you mean in the Serpent Temple...There are four keys you need.

Heretic's Key -> from Kol in front of the temple, used to find the Kerah Statue
Serpent Key -> from Kreug in the temple, used to open the Serpent Holds (and get the Serpent Idol
Monkey Key -> from the Inquisitor, used to open H'Thark's jail cell
Skull Key -> from the Serpent Master at the bottom of the "lift maze", used to open A) the room where the Tome of Fire is and B) the passageway to Elyssia

I'm not sure which key you're referring to, so I've listed them all here. Hope this helps.

-Sazerac