Arc-Gurren Lagann

"Breaking through Destiny and the Milky Way, the screams of life echoing through the galaxy! The union of Raging Billows, ARC-GURREN LAGANN!"

- Simon and Viral

Arc-Gurren Lagann (アークグレン, Aku-Guren-Ragan?), taken from the name Arc-Gurren given by Rossiu Adai, is the combination of one of Lordgenome's Gunmen battleships with Simon's Gurren-Lagann. It is formed when Simon combines the two mechs in order to save the former from destruction at the hands of the Anti-Spirals' attack force, the Mugann. It is later used as the Ultra Spiral Powerplant that powers Super Galaxy Gurren Lagann.

Before Anti-Spiral War Arc
Simon and the rest of Team Dai-Gurren appear to fight back against the Mugann onslaught attacking Arc-Gurren. When Arc-Gurren is cornered by two large Mugann, Leeron tells Simon to combine it with Gurren-Lagann, which causes Team Dai-Gurren to form Arc-Gurren Lagann. The resulting combination is able to knock out the two Mugann, who combine as well, and create a hole in the space time continuum.

After the battle, the Arc-Gurren Lagann was used to hold back the moon as it tried to crash into earth ahead of schedule, and when it was unable to push it back, Gimmy Adai and Darry Adai took its controls using their Grapearls while Simon used Gurren-Lagann to take back the moon. In the movie adaption, Arc-Gurren Lagann instead breaks the hand of the Cathedral Lazengann with its own Giga Drill Break and the mech itself pushes its Giga Drill into the moon's core.

During Anti-Spiral War Arc
The Arc-Gurren, in the form of Arc-Gurren Lagann, is only seen outside of the Cathedral Terra when Team Dai-Gurren first enters the Anti-Spirals' pocket-universe. Once it becomes the Ultra Spiral Powerplant that powers Super Galaxy Gurren Lagann, it is only seen acting as such within the mech until the end of the series. In the movie adaption, it is only seen acting the Ultra Spiral Powerplant as well, and again only until the end of the series.

Attacks
Space-Time Shattering Burst Spinning Punch: A punch used by Simon and Viral which tears a hole in the fabric of reality. The move was used to defeat the combined form of two large Mugann by sending them to the far end of the space time continuum, where they exploded.

Arc-Gurren Lagann Giga Drill Break: A move only used in the second movie. Arc-Gurren Lagann performs a Giga Drill Break, which tears apart one of the arms of Cathedral Lazenngann before it can punch the Earth.

Trivia

 * It is the only Gurren Lagann form (apart from Lagann itself) to lack sunglasses on either face.
 * It's the third (technically fourth if you count Super Tengen Toppa Gurren Lagann) strongest mecha in the series, it can rip a hole through reality in a single punch.
 * It's name could presumably be rendered as Ark Gurren-Lagann, as a reference to Noah's Ark, which was also a giant ship used by humanity to escape a major disaster (flood, moon drop).
 * Arc-Gurren Lagann made a cameo appearance in parallel works, "Kiyal's Magical Time, Three Minutes Before", being piloted by Kiyal to stop a huge ship (which far larger than it) from crashing into Earth. This may have been a reference in its debut where Simon used it to stop the Cathedral Terra from colliding with the Earth.
 * Arc-Gurren Lagann shares design elements with Linebarrel (Linebarrels of Iron) and Jehuty (Zone of the Enders), namely the legs, the spine, and the lower torso.