Robofusion

Robofusion is a special ability that can allow the Tenkai Knights or the Corrupted to combine each with other to form giant vehicles that are more powerful than their singular forms.

Tenkai Knight's Robofusion
Protojet

The Protojet (Japanese: テンカイジエツトフアイヤ―バ―ド, Tenkaijietsutofuaiya-ba-do) is the Tenkai Knights' first Robofusion that resembles a giant battleship, armed with pieces and colors from Bravenwolf and Tributon. Its special technique is the Phoenix Attack.

Air Lancer Jet
The Air Lancer Jet is the second Robofusion that the Tenkai Knights have achieved. It is armed with the color of Valorn and Lydendor. Its special technique is the Tenkai Thunder Cannon.

Corrupted's Robofusion
Xenoship

The Xenoship (Japanese: ヴィルバット, Virubatto) is Vilius's Robofusion with three of his soldiers.

Sickle Ship
The Sickle Ship is Granox & Slyger's Robofusion with a Shadius and a Sho. It's special technique is the Dark Raven Attack and can also move while underground.

Flying Darkwing Spector
The Flying Darkwing Spector is Rho's Robofusion with three different looking Hos's. It was destroyed by Valorn in "Toxsa 2.0" and made a reappearance in "Double Crossed".

Titan Fusion
Titan Fusion occurs when two members of the Tenkai Knights robofuse while in Titan Mode.

Slyger and Granox appear to perform a variant of Titan Fusion, although they robofuse with a mech beast (a Sky Griffin and War Stallion, respectively) and have only done so when they have been empowered with energy from the Tenkai Dragon.

Mega Fusion
Each one of the Guardians can fuse themselves with lots of Corrupted soldiers, transforming themselves into giant robots.

Trivia

 * The Xenoship and the Protojet are both able to be made by connecting four different toys each. (Protojet - Tributon, Kutor, Leinad, and Bravenwolf and Xenoship - Vilius , Rho, a Hos, and a Sho)
 * All current Tenkai Knight Titan Fusions incorporate Bravenwolf, but only one has him as the upper body.
 * Dromus and Venetta are currently the only Knights that haven't used Titan Fusion.