Bringing back Diamondhead was a major "fan service" moment that integrated the original series' lore into the Alien Force era.
Knowing his current roster isn't strong enough to stop this version of Vilgax, Ben ignores Azmuth’s warnings and attempts to to unlock the Master Control. The Turning Point: The Hack Gone Wrong [S3E2] Vengeance of Vilgax: Pt.2
This wasn't the cyborg Vilgax of the past; this was a "Conqueror of Ten Worlds" with a vast array of powers, including flight, super strength, and wind breath, making him a more versatile threat than ever before. Bringing back Diamondhead was a major "fan service"
This episode signaled a darker, more consequence-heavy tone for Season 3. Ben’s arrogance in trying to hack the watch led to Kevin’s mutation, a burden Ben carries for a long time. This episode signaled a darker, more consequence-heavy tone
"Vengeance of Vilgax: Part 2" is the high-stakes conclusion to the two-part premiere of Ben 10: Alien Force Season 3. It marks a pivotal shift in the series, featuring a desperate power play, a significant change to the Omnitrix, and the return of Ben’s most iconic nemesis in his most powerful form. The Premise: Ben’s Back Against the Wall