The Monarch Of Death

[You have successfully formed a contract with the guardian of Taurus!]

With a soft smirk, Rentaro stood firm, glancing down at the Mobius Titan King.

"I'll show you the true power of a Dragonmancer, Summoning — Crader."

The eyes of the guardian of Taurus glowed a bright red as Rentaro gained control of the guardian of the underworld.

[You have successfully sealed a contract with the guardian of Taurus!]

[You have levelled up!]

[You have gained a new skill!]

[Dragonic Order - Level 1]

[You have levelled up!]

The guardian of Taurus, Crader, is the guardian of the gates of Taurus, the gates of Taurus is basically the prison where the Primordial Demons were cast into.

The guardian of the gates, Crader, possessed power over the dead, basically, it can resurrect the dead momentarily and gain control over the undead. Which explains why the Mobius Titan that died suddenly came back to life.