Shiki

Shiki is the Death Force Energy used by Death Gods and other characters. It's the complete opposite of regular Ki which is Life-Force Energy.

Achieving Shiki
To acquire Death-Force Ki (Shiki); one has to transcend death itself which is easier said than done. To transcend death one must train under the tutelage of the current Death God or try to escape the Otherworld and drink from the first Teapot followed by a few additional steps.

Teapot Method
After drinking from the first Teapot; the user will regain their body to use in the living world for a limited time and must meditate under a solar eclipse before meditating under a lunar eclipse.

After meditating; the user will need to drink from the first Teapot for a second time but only with a specified ingredient which is different for every drinker. After drinking the mixture; one must go through process of learning to harness ki but doing so require immense concentration. Once the user's ki turns black; they've done it and have achieved Death-Force Ki.

The specified ingredient is usually the object that the user has used the most in life.

Death Gods
Death Gods can teach their students to use Shiki and is usually taught to souls sent to Hell that wish to have repentance. However, Shiki taught by a Death God can be unstable unless the user learns proper Shiki control.

Nether Energy
Users of Nether Energy are capable of using Shiki and have a fine control over the power.

The First Teapot
The First Teapot is as it seems; the first ever Teapot to be created. Due to the dangers of Shiki; it is often guarded by a phantom Supreme Kai which orders them to live the worst days of their lives and must remain determined to acquire the Teapot.

Death Gods
Death Gods like Shroom are the most common users of Shiki and usually have Master Shiki Control allowing them to use the technique Death.

Other
Other users include the Three-Star Dark Dragon who uses Death-Force energy via his Nether powers.