Robofusion

Robofusion (Japanese: ロボフュージョン, Robofyūjon) is a special ability that can allow the Tenkai Knights or the Corrupted to combine with each other to form giant vehicles that are more powerful than their singular forms.

Protojet
The Protojet (Japanese: テンカイジエツトフアイヤ―バ―ド, Tenkai Jietsuto Fuaiyabado, Eng. Translated: Tenkai Jet Firebird) 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 (Japanese: サンダ―バ―ド, Sandabado, Eng. Translated: Tenkai Jet Thunderbird) 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.

Phoenix Protojet/ Phoenix Jet

The Phoenix Jet is the Tenkai Knights' third Robofusion. It is basically the Protojet in Phoenix Mode. It resembles a giant battleship, armed with the colors of gold and yellow. It was first seen and used in "A New Knight" to travel to the Guardians Sanctuary to fix the malfunctioning portal.

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 (Japanese: ブレイガ―, Bureiga) is Granox & Slyger's Robofusion with Shadius and a Sho. It's special technique is the Dark Raven Attack and can also move while underground.

Flying Darkwing Spector/ Triberus
The Flying Darkwing Spector (Japanese: フライングキラ―ズ, Furaingukirazu) is Rho's Robofusion with a Gatox, a Gearnox, and a Gilltax. It was destroyed by Valorn in "Toxsa 2.0" and made a reappearance in "Double Crossed".

Titan Fusion
Titan Fusion (Japanese: XXモード, XX Mōdo, Eng. Translated: XX-Mode) 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.

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

Trivia

 * The Xenoship, Protojet, Air Lancer Jet, Sickle Ship, and the Flying Darkwing Spector are all able to be made by connecting four different toys each. (Protojet - Tributon, Kutor, Leinad, and Bravenwolf, Xenoship - Vilius, Rho, a Hos, and a Sho, Air Lancer Jet - Lydendor, Valorn, Senjo, and Tavox, Flying Darkwing Spector - Viklips Trooper (Rho), Gatox, Gearnox, and a Gilltax, and the Sickle Ship - Granox, Slyger, Shadius, 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 or Robofusion.