The Maker's Key is an item of immense power created by the Makers.
Prior to DarksidersEdit
The Makers created Maker's keys to aid in the creation of their Constructs. It takes a fragment of the soul of its user and inserts it into the body of a construct, acting as a conduit for the transfer and bringing the being of stone to life. It would seem that all the keys the Maker's had already made had been destroyed by Corruption, as there are none when Death arrives in the Forge Lands.
When Death arrived in the Forge Lands in need of a way to reach its Tree of Life, the Makers suggested finishing the Guardian, but to get to the Guardian and complete work on it Death will need one of the Makers keys, and to make one they need to restore both the Drenchfort and The Cauldron to gain access to the necessary materials.
Once Death has completed the errands the key is made by the Makers Alya and Valus and given to Death, which soon proves invaluable to his travels through the Forge Lands as he activates Custodians to aide him and restores Wardens to guide him through his journey. Unfortunately, the key doesn't retain its usefulness outside of the Forge Lands, as only there does Death encounter Constructs to activate.
Function and UseEdit
The key is a invaluable tool for anyone who works with or creates constructs: it allows the user to inject a portion of their soul into a stone body to bring it to life (it is unknown at this point in time if there are any long term consequences for this, as one is literally inserting a piece of their life-force into a stone being).
Death uses it to activate Custodians that he can then control as a basically invincible juggernaut (for a limited time, as the large Custodians are often found in rooms that they are too big to fit through the doorways of) that can smash corruption or enemies with its bare fists. It can also use chains to form tightropes across gaps that Death can then use.
The Maker's Key can be considered the Darksiders II equivalent of the Earthcaller in that both can be used to activate Constructs. However, The Maker's Key is only useful in the Forge Lands and has no combat use on its own, and the Earthcaller is usable all across Darksiders, being highly useful in temporarily disabling the Phantom General.