Soul Cube/Doom (2016)

The Soul Cube appears as a background object in both games set in the Doom 2016 timeline. It appears to simply be present as a Easter Egg reference to Doom 3: it cannot be picked up or used, and there is no storyline information associated with its presence.

Doom (2016)
The Soul Cube is present on a desk in Olivia Pierce's private office in the level Lazarus Labs.

Doom Eternal
The Soul Cube is found on the Slayer's main computer desk inside the Fortress of Doom in Doom Eternal.