Socketable

The Trialmaster will also occasionally offer you Soul Cores.

Originally formed by Chaos, the Vaal sought to replicate Soul Cores through human sacrifice rituals and blood magic to power their civilization.

Soul Cores are powerful socketable items with mods that cannot be obtained from regular runes.

Like all endgame content, there is of course a pinnacle boss. The final chamber will drop keys to this mysterious door.

Some say this is where the Trialmaster himself resides, and perhaps you're willing to take the risk and find out.