Writhe. Scientist. Inventor. Invented the cloak he wears that gives him his powers. His bio indicates that the cloak malfunctions and fuses with him to a degree.
Writhes incap side depicts him by the belltower. This presumably is the same belltower we see the Seer standing in front of, and most likely the tower than Fanatic holed herself up in after her first battle with Apostate before becoming the "Redeemer". He seems to be loosing control of the cloak. His body is morphing. Look at that hand...it looks nasty.
Setbacks card art for "Cause and Effect" show Setback being attacked by a black ink-like creature. It looks formless and fluid. It looks a lot like Writhes incap side.
Setbacks incap art is straight from a Spiderman panel where Peter Parker ditches the suit to try and live a normal life.
Comic fans might know where I'm going with this. Going with the Spiderman frame here, Writhe being shown loosing control in front of a belltower is telling. Venom, the alien symbiote, also got its start in a belltower. Tell me the black evil thing on "Cause and Effect" is not a dead ringer for a Venom-like creature.
We don't know the full story here, but it is looking like at some point Writhe looses control of his cloak and it turns him into a monster. Looking forward to a Sentinels promo in the future!
UPDATE EDIT:
During conventions, I make it a large point to not bother Christopher at all with story questions. Mostly because he has a ton of more important things to do at these conventions than pander to my slight obession with his story. However, I managed to sneak in one small question on the final day of PAX East (I couldn't help myself. Sorry Christopher. I tried being good). Anyway, I basically asked if the evil black thing on "Cause and Effect" was Writhe. To my surprise, I got a straight answer.
"No"
Awesome. That's all I needed.
I am not at all prepared to give up the Venom/Belltower connection here. The art and Spiderman themes seem far too intentional to just throw the entire thing away. Plus, we have new art from promo Writhe that needs to fit into this working theory. Let's start with promo Writhe. The biggest difference with Writhe here is that he has lost the form of his cloak that initially fused to him which gave Eugene his powers. That is interesting.
Follow this new theory: Writhe's incap art shows him losing control of himself. It's a theme with all of the Sentinels original incapp art. Mainstay loses control of his freedom (being tied up and captured), Medico loses control of his body (he is a pile of dust), Idealist loses control of her mind (she is systematically coming apart), and Writhe is losing control of his powers (seen with the ugly transformations). In Marvel comics, Venom is a symbiote, something that takes control and fuses with a host. We even see Venom as an amorphus blob of horror exist outside of it's host a bunch in the comics. Eugene's cloak has this same sort of thing going on, having a symbiotic relationship with Eugene and fusing with him.
"Cause and Effect" shows Setback being attacked by something. But it's not Eugene. It's not Writhe. It is the "Cloak"
I now suspect that his incap art is showing a point where the "Cloak" is trying to separate itself from Eugene. And I think it does and gains a mind of its own. Maybe the shadow energy always had some free will component, maybe the energy fused with the darker aspects of Eugene's psyche, but it's coming loose. Maybe Eugene was going off the deep end, the shadow energy taking it's toll on Eugene, and he starts getting desperate and doing some less than heroic things. Something along these lines could explain why we see Tachyon fighting Writhe right? During the scene at the Belltower, the good side of Eugene could be fighting against the darker side of him, represented by the cloak. I think ultimatly the two sides split. That's why the Writhe we see in the promo art has lost the figure of the cloak and is a fully commited member of the hero team The Sentinels and has retained a large portion of his powers because of the initial fusion.
And the "Cloak" roams free, possibly looking for a new host, or possibly it just wants to be evil. I don't personally think there is a connection between "Cause and Effect" and Setbacks DW costume change to all black, but my new theory makes it more plausible.
(Possibly just as a character in someone else's deck?)
" in there.