The Ankh is an item of equipment that can be be acquired from a certain shop the Black market. It always costs a set price of $50,000 if purchased, though it can be stolen at great risk.
It is necessary to buy this item if you want to have any hope of reaching the City of Gold.
Functionality
The Ankh does one thing and one thing only - it breaks when you die, resurrecting you with four hearts.
Although this power probably makes the Ankh one of the most useful items in the game, you will most likely have to utilise it early in order to reach the City of Gold, as you MUST consume the Ankh in one specific level of Area 3. You will know which area this is when you see a giant Moai with an ankh on top of it.
You must kill yourself within a certain distance of the Moai for this to work, and should avoid using a bomb, due to the risk of aftershock effect killing you again as soon as you respawn.
The best method of resurrecting safely is to simply press Escape and hit F1. Successfully using the Ankh on the Moai level will cause you to respawn inside the giant statue, where the Hedjet is lying on the floor in front of a secret exit.
If you are currently wanted by the shopkeepers, you may wish to be in sight of the Moai when killing yourself, as alternatively, a shopkeeper may appear inside the Moai, killing or disarming you.
Should you decide to bypass the City of Gold and keep the Ankh for later levels, you must still avoid lava in Area 4, since falling in will melt the Ankh before it can revive you, meaning instant death.
When reviving yourself with the Ankh, you will retain your current Money, Equipment, and Accessories. Held items (such as a Shotgun or a Rock) are usually left behind when you die, but are always teleported with you if you commit suicide.
Trivia
The Ankh is an Egyptian symbol representing eternal life - appropriate for an artifact that can raise the dead.