"I popped his eyes and drank the goo"
(Skylord Vitali on the Skylord Unknown's death)
The unknown Skylord was one of the Skylords murdered by rogue Skylord Vitali. Vitali popped his eyes out and drank the goo. His remains lie under an apple tree on Skyhold, along with four other Skylords. Out of all the Skylords Vitali killed, he suffered the most gruesome death.
It could be assumed that Skylord Unknown is Skylord Valetius. This could be said by how when Skylord Valetius appeared in the Yogscast Tekkit series (Duncan's Laboratory) it was two years after Skylord Vitali's rampage on the Skylords (after people's debates on Skylord Unknown). So it could be assumed that Skylord Valetius was featured in Tekkit for us to speculate whether Skylord Valetius is/was Skylord Unknown. And as Tekkit is on a totally different server to the Shadow Of Israphel server it doesn't mess with the storyline at all. Reasons that this could be plausible; Skylord Valetius wear's purple googles over his eyes that could give the impressions that they could be purple from blood mixing with black, and so his eyes could have been gouged out. And he was seen after fans realisation on a missing identity of a Skylord.
Also another theory that could have an even bigger explanation for why Skylord Valetius could be Skylord Unknown and why he appears in Tekkit could be that he is going make an appearance in the Shadow Of Israphel series.
Another theory is that Skylord Unknown did not exist in the first place. The poem of death, written by Skylord Vitali, read:
I stuck a meat hook in his back,
I snapped his spine with a mighty crack,
At morning light our swords did clash, nothing left but bones and ash,
I popped his eyes and drank the goo,
I took his blade and ran him through.
"At morning's light our swords did clash, nothing left but bones and ash" is the line referring to Skylord James. Therefore, it may be possible that "I popped his eyes and drank the goo, I took his blade and ran him through" may refer to Skylord Finnigin, another victim, as the two lines seem to be paired up. If this is the case, then the Unknown Skylord does not exist, and the two lines both belong to Skylord Finnigin, whose remains are next to the Unknown Skylord.
In a poll that was held from January to February 2013 on this wiki with all Season 3 characters to choose from, the Skylord Unknown was placed equal 26th overall with 5 votes out of 4412 votes.