"The time of Chaos, the era of cosmic creation that lies in Borges. Awaken now! This is Chaos, the power of the Malicious Demonic Monster Grimdo!"
- ―Ultraman Tregear, The Divergence.
Subtitle: Malicious Demonic Monster (邪神魔獣 Jashin Majū)
Long ago, before the creation of the universe itself, "Grimdo" existed as the form of primordial chaos gods, who were sealed away in the graveyard ruins of Planet Borges to maintain the balance of the universe, though ancient cosmic runes were left behind. There was similar ancient writing in a cave on Planet Tika-Du, where Ultraman Tregear and Ultraman Taro ventured to during their youth. After Tregear's fall to the darkness, he managed to decipher the ancient runes on Planet Borges and unsealed the ancient chaos gods, who were absorbed into his Color Timer, granting him great power. Tregear covered his Color Timer with strong restraints to prevent Grimdo from escaping.
Ultraman Taiga The Movie: New Generation Climax
A part of Grimdo was freed from Tregear's body when the seal was loosened after the battle between Tregear and Ultraman Taiga. The monster is sealed by the New Generation Heroes for a short time at the cost of their transformation abilities, and made its way to Earth. It escapes its seal and wreaks havoc on Earth, with Taiga, Ultraman Rosso and Ultraman Blu unable to contend with it. As part of Tregear's plan, Taro showed up to help his son and used the Ultra Dynamite on Grimdo, though the monster possesses him in the process. Taro, under the influence of Grimdo, fought against Taiga and forced him and the brothers to retreat.
Grimdo leaves Taro after he is saved by Hiroyuki Kudo and the Tri-Squad, who clash into him using the Ultra Dynamite. Their combined power managed to drive out Grimdo from Taro's body and restore him to his usual self. Having lost everything, Tregear decided to offer himself to Grimdo. His body broke down and was absorbed into the beast, restoring it to its full power. His consciousness was within the monster and controlled it, with a desire to destroy everything.
The Tregear-Grimdo fusion overwhelmed Taro and the New Generation Heroes, even in their strongest forms. Taro advised them to fuse via Taiga's Ultra Horns while he distracted Grimdo, allowing them to have the opportunity to fuse into Ultraman Reiga, who was strong enough to deflect all of Grimdo's attacks. At the climax of the battle, Reiga shot through Grimdo's central eye with the Reiga Ultimate Blaster. With a sorrowful voice, Tregear called out to Taro before he is destroyed along with Grimdo.
- Designer: Masayuki Gotoh
- Height: 70 m (Freely set)
- Weight: 70,000 t (Freely set)
- Origin: Beyond space and time, sealed on Planet Borges
- Powers and Weapons
- Grim Howling (グリムハウリング Gurimu Hauringu): A shockwave unleashed by roaring loudly.
- Grim Bolt (グリムボルト Gurimu Boruto): Grimdo can fire red lightning bolts through its horns and back.
- Grim Ray (グリムレイ Gurimu Rei): A destructive beam fired from Grimdo's central eye. It is very powerful, but even at its peak it was still overcome by the Reiga Ultimate Blaster. It can summon several eyes in its giant form to also fire the beam.
- Portal: Grimdo can create a portal to travel through.
- Beam Negation: Grimdo has the ability to negate an Ultra Beam and absorb its energy, seemingly without taking damage.
- Acceleration: Grimdo can accelerate to great speeds.
- Possession: Grimdo is able to take over the body of other beings.
- Empowerment: Grimdo can grant other beings and Kaiju a portion of its own power.
- Absorption: A purple beam that caused Tregear's body to break down and be absorbed into Grimdo's mouth to restore it to its full power.
- Size Increase: At full power, Grimdo can increase it size seemingly infinitely. It also changes the surroundings into a chaotic dimension-like area.
- Gigantic Body Parts: Grimdo can conjure gigantic body parts separate from its "main" body, used to crush an enemy.
- Lightning Beam: Blue lightning beams fired from the hands. Only used after absorbing Tregear.
- Durability: At full power, Grimdo can withstand getting hit by the New Generation Heroes' consecutive finishers without taking any damage, with only Ultraman Reiga being able to damage and destroy Grimdo.