You cannot use Golden Key on the lock (tested).
The room just behind Tigo1 and before Leiden room is the "treasure & wealth dump" room. As you can see room contains several different items from other parts of Tibia- Ankrahmun, Zao etc etc. Golden Lock Pedestal is item left (or maybe build also) by one of those who attempt purification. There is possibility it is lock protecting from entering the throne room and Leiden’s chamber without being purified yet. Whoever was joining the Cult, was “stripped” of from their material goods as they are not what really brings us happiness. Simplicity (which can be seen in throne room), humility and modesty (trials) are main goals of the Cult and suffering helps to achieve it (this is why healing Leiden kills him;-).There are several preparations and tests before main purification (all bringing suffering and even death to your character). All those tests are to ensure that you ready to accept and survive the final purification. Every Trial brings some kind of cleansing. We as players - are unable to get purified and we share fate of Telia (who was beloved of Tigo and died due to having feelings for him- which was forbidden among Barkless). The difference is that we are able to return to the Tibian world unharmed.
Golden Lock pedestal is yet another indirect connection to Variphor4- Golden lock pedestal2 is the one which gives us access to Prince Drazzak3 – Minion and follower of Variphor. Cults of Tibia quest is about Cults of different races, and their leaders been corrupted by Variphor. Roshamuul Prison was build by the Dream Catchers & other races- which also included elves. In the books which we can find in Barkless spawn, Tigo says Leiden been corrupted and is not himself anymore (which makes us assume Variphor corrupted him). He stays in his chambers (the boss room is his chamber, the throne room is behind the treasure room) and he lets down the Cult.
Tigo: The Penitent is the first and truest of the Barkless. Our spiritual leader. He walked the face of this world in suffering and pain for us to be freed from misery on the other side. ...
Tigo: Only he can judge if our brothers and sisters are pure enough to approach him.
Tigo: When you are free from vanity and fortune, you shall receive purification. Then, and only then, the Penitent will judge you. Survive the trial of tar, sulphur and ice.
Tigo: Purification is but one of the difficult steps on your way to the other side. The trial of tar, sulphur and ice.
Tigo: The trial consists of three steps. The trial of tar, where you will suffer unbearable heat and embrace the stigma of misfortune. ...
Tigo: The trial of sulphur, where you will bathe in burning sulphur and embrace the stigma of vanity. Then, there is the trial of purification. The truest of us will be purified to face judgement from the Penitent. ...
Tigo: To purge your soul, your body will have to be near absolute zero, the point where life becomes impossible. ...
Tigo: Something about you is different. I know that you will find a way to return even if you should die during the purification. And if you do... Leiden will become aware of you and retreat. ...
Tigo: If he does, follow him into his own chambers. Barkless are neither allowed to go near the throne room, aside from being judged, nor can we actually enter it. ...
Tigo: He should be easy to defeat with his back to the wall, find him - and deliver us from whatever became of the Penitent.