Serus

The god of power, Serus once ruled the 7th universe until his exile by his father Oerus. Serus now plans to seek havoc across the universe and Beerus or any of the other gods cannot stop him. It all comes down to a Super Saiyan. ~ Dragon Ball Z - Battle of Gods (Series)

Invasion of Earth
Serus attacks planet earth on his rampage across the universes, meeting Goku. Goku attempts to fight with Serus but is easily beaten. Piccolo arrives and senses the possible power this god may have. Piccolo becomes shocked and eventually flee's to dende's lookout in order to plan an attack against this powerful god. Despite fleeing wasn't the best option, it is later revealed that Serus never learnt how to sense other's power levels.

Serus tries to locate Piccolo and ends up destroying most of the towns until Goku, Gohan and Vegeta eventually catch up to him. Goku and Vegeta both become Super Saiyan Gods using the help of Gohan's energy. After a brawl with Serus. Goku and Vegeta have no other options but fuse into one being. Super Saiyan God Gogeta. The Saiyans try to fuse but is interrupted by Serus's Energy Canon which beats both of them. Gohan carries Goku and Vegeta away from the fight back to Dende's Lookout while Serus follows close behind. Serus is then kicked into a nearby debris allowing Gohan to successfully get to the lookout without being tracked.

Once at the lookout, Goku and Vegeta are given senzu beans and train as Super Saiyan Gods in the Hyperbolic Time Chamber when Mr. Popo tells them that a god can survive infinite time in the time chamber without taking any effect.

After 4 Hyperbolic Time Chamber years (Four earth days), Goku and Vegeta emerge with a much higher power level. Vegeta's arrogance consumes him as he chases after the opportunity to fight Serus alone. Goku chases Vegeta in attempt to make him reconsider.

Vegeta tracks down Serus in a village. Serus laughs at Vegeta and attacks him, Vegeta transforms into a Super Saiyan God and dodges most of Serus's attacks until he is pummelled into a boulder. Goku strikes Serus straight after delivering a devistating blow. Serus is not weakened by this kind of attack and fights with Goku and is proven still not a match. Vegeta emerges and stops the fight. Vegeta then transforms into a Super Saiyan 3 God.

Vegeta has a long fight with Serus with their powers being equal. Serus has the upper-hand in speed which he uses to defeat Vegeta. Goku steps in, transforming into a Super Saiyan 3 God and fights with Serus ending with a Gogeta Fusion which destroys Serus.

(This Plot is showing the most interesting parts, while Goku and Vegeta were training in the HTC...Gohan fought with Serus and Piccolo and the others made an attempt to stop Serus from attacking the nearby towns and powering up to a force that would destroy the planet).

Transformations
Serus has 3 transformations. The first one allows his body mass to increase which increases his strength. This transformation is used against Vegeta (Super Saiyan God) and powers up his ultimate attacks. The second transformation changes his look completely...instead of getting bigger, Serus gets lighter and a lot faster which allows him to dominate his opponant and also deliver devastating attacks. The third and final form consists of the use of both. A body mass increase and really agile movement. This is the ultimate god form which defeated Vegeta and Goku in Super Saiyan 3 God with ease. Until it was overpowered by Super Saiyan 3 God Gogeta.