You need the 3 stones that you have collected in the Defiled Desert. See "Defiled Desert spirit" to know how to collect them. Of course, you already had a conversation in the Desert using them...

Bring the stones to Clockwork island 3 (the one in SW). You will also need at least 2 amulets to get there, in order to cross the underground barriers.

Enter the 3-stones (the little stones) triangle which lay on the ground, south of island. This will automatically start the conversation with the spirits.

If it's the first time you do it, you will collect some XPs reward.

Each time, you will also get some magic item, possibly a UNIQUE (if available). It's probable that the only UNIQUE you have here is a necklace of undead control (I had it several times here), that might be useful when Mastaba is freed.