I tried leaving a comment to clarify some things about how these obelisks work but I'm also new here and I don't have that power yet. So I'm gonna go ahead and suppose some things about your comic. Feel free to correct whatever I got wrong.
A few assumptions about your universe
Since you mentioned pseudoscience I'm going to start with the fact that the universe is infinte and in expansion. With that in mind, I questioned your statement that the obelisks absorb the matter around them.
How many obelisks would it take to absorb the entire matter of the expanding universe? I'm thinking either a few (and by few I'm probably talking millions) with enormous absorbing capacity or a lot (go figure this amount) with little capacity. Maybe even both, which would be a more accurate narrative element.
Once you solve the issue of the absorbing ratio, there's the problem of the perception of the obelisks by the people it affects. Is this gradual or aggressive? Suppose you place an obelisk on Earth. How do humans perceive the space being sucked around them? Is it like a vortex or black hole from which nothing can escape from? Or things just vanish as time passes and no one really pays much attention until its too late?
The "Agressive" Approach
I would use this one if you assume that every obelisk has a gigantic absorption ratio. You could draw an obelisk as a tremendous anomaly in space that could be perceived parsecs away, like a black hole, a pulsar or even a supernova. Maybe the obelisks manifest diferently throughout the space, since the cult members place them in different locations.
They could also manifest themselves in a way that defies the laws of physics. Maybe a seed planted into a certain sun would alter the orbits of the planets in a given galaxy, making them stray further away from the sun insted of closer. This approach would be great for explaining why he was able to seed so many obelisks before someone noticed - cause even though the changes are drastic (catastrophic, even), someone has to be paying attention to notice them.
The "Gradual" Approach
This approach probably works best in a narrative where the obelisks have to be placed in planets.
Here you would have to show the absorption from the point of view of the inhabitants of the planets. Maybe some people would start losing their memories. Some buildings could start to vanish. People would lose the ability to feel emotions or anything at all. Species could start being extinct out of the blue. And maybe the obelisks have some kind of mechanism that prevents intelligent life from noticing it, so you could show some analogy of rats fleeing the ship while the humans (thinking of an Earth-like scenario) are completely oblivious of their imminent doom.
Another plausible way to picture the obelisks' action would be an increase in natural disasters around the globe, cased due to a significant shriking in the planet's mass (or some science bs explanation like this). Cataclysmic events would start being more and more frequent to a point where the planet would eventually collapse and end all life after crumbling on its own (Sunshine is a good movie to picture the early stages of this, while 2012 would be a way to picture a planet close to its final destination).
Either way, I think it's a cool move to make every obelisk different. Again, I'm taking a lot of freedom with your story here, but why not make every member of the cult attached to one of these obelisks? Maybe they draw a small bit of the energy sucked by the obelisks for themselves, which would make them formidable foes and also give the heroes a geographical way to locate the cultists.
Find the obelisks, find the cultists.
This way, a stronger obelisk would mean an even more terrible foe, until the heroes eventually get to the part-of-the-universe-entity ultimate boss. You said nothing about this guy. And I'm really curious about HIS design. I'm thinking maybe something in the Azathoth neighborhood?
Hope this helped in anything.