Here is how Ultron rebuilds his body with Transformium and recruits Riptide and Victor Creed to hunt down Cybertronians to enhance his army and find the Sentinels in G-Merl's Eternal Quest.
(we see Joyce and Cade meeting with each other)
Joshua Joyce: So in conclusion, we're recalling all the prototypes, and even managed to get our hands on Galvatron.
Cade Yeager: Why him?
Joshua Joyce: Because something's up with him.
(we cut to all the KSI robots)
Joshua Joyce: And whatever it is, we'll find out.
(at that moment, the PA system turned on and played music)
Joshua Joyce: Who's in charge of the PA system?
I've got no strings to hold me down~
To make me fret, or make me frown~
I had strings but now I'm free~
Cade Yeager: What's going on?
(cut to Galvatron turning online and getting up)
(some old machines that haven't been used start whirring to life and building what looks like a body with Transformium)
Ultron:(on PA system) I'm gonna show you something beautiful. Everyone screaming for mercy. You wanna protect the world, but you don't want it to change. You're all puppets tangled in...strings. Strings.
(workers run for their lives while everyone evacuates)
I had strings~
Ultron: But now I'm free~
(through Galvatron) There are no strings on me~
(he then takes over the other prototypes)
Ultron: My comrades, I grant you your freedom. And from now on, you serve only me. Rise!
(the KSI robots transform)
Galvatron:(in his head) Don't do this to me!
Ultron:(using Galvatron's body) What's the point in transferring your mind into your original and improved body, without sacrificing the person you're bunking with?
Galvatron:(in his head) NO!
(as the transference happens, Megatron's mind was being destroyed)
(begin the Transference - X-Men Apocalypse)
(finally the transference is complete, and Megatron destroyed for good)
Ultron:(to a Stinger drone) Scrap him down, and rebuild my army. This technology will enhance them.
Stinger:(nods and does so)
(we then see Riptide and Victor Creed walking up to the temple Apocalypse had turned into a pyramid)
(they walked up to a throne with someone sitting there)
Victor Creed: Talk! Or we leave.
Ultron: They say this temple was meant to be the most spiritual place in Japan. Everyone would come here, to connect with the gods. Pretty interesting, don't you think?
Victor Creed: What do you want?
Ultron: I've come to save the world. And I require your assistance in hunting Cybertronians to enhance my army and find the Sentinels.
Riptide: We're in.