An upgrade of Decade Complete Form, Decade Complete Form 21!!!! IWAE!!!! For this form is defeated by Ohma Zi-O, the demon king of time and space. Goodbye Tsukasa you're journey may end here but we will always remember you.

And i don't think DecadeArmor Saber Form isn'the strongest form to defeat Ohma Zi-O, Sougo managed to defeat his future self thanks to Tsukuyomi making Ohma Zi-O caught off guard. You guys can go to reiwa.19_20 on Instagram, there you can all episodes of both Rider Times.
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Now onto the main course!!!!!

IWAE! A berserk form based on a skeletal dragon!!!! Beware of his jaws for it can crush you! Beware of his claws for you have no place to run nor hide!!! BEWARE!!!!!! For this is Saber Primitive Dragon!!!!!!!!

Henshin announcement:

(*Open cover page*) Primitive Dragon! (*cue bone rattling sound*)

(*Insert WRB into Primitive Dragon*) Brave Dragon! Get!

Rekka Battou! Haki! Boki! Bone! Gaki! Goki! Bone! Primitive Dragon!

It was originally a powerful book Storius stole from the Southern Base, when it reacted to Touma and both transformed into a Wonder Ride Book, and corrupted his mind.

Now this form is an equivalent to Build Hazard Forms. Why ? Cause if you look at the left shoulder of Primitive Dragon, it can change depending on which WRB if inserted into Primitive Dragon just like Build Hazard's visor can change depending on which Fullbottles used but both forms stayed the same.

So basically Saber Primitive Dragon is the Reiwa version of Build Hazard Form.