This is the one question that been bugging me for a while. We've all heard or read the lore and at some point Perturabo becomes a demon prince, the question is how and when?

Perturabo had no pacts to any of the gods of chaos. So how?

IIRC he sacrificed a bunch of captured Imperial Fists' geneseed to the Dark Gods for power, and thus ascended.

7154186
What book is this from?

7154189
I don't remember, unfortunately.

7154189
it was said in the book one of the index Astratus, the whole campaign at the Fortress of Eternity was a trap to lure the Imperial fists there and their was nothing that was actually being protected or defended like what was the Fists' first assumptions.

7154192
Hmmm, i'll need more details. Like how and why? Just becoming a demon prince seem a bit short sited to me.

7154208
Rogal Dorn picked a clue about the endeavors of Perturabo and swore that he would "bring Perturabo back to Terra in an Iron Cage"... well, there was a world named Sebastus IV (i don't remember if it was a fortress world, probably) where Perturabo lured Dorn and a great number of his legion (If i recall correctly, it was before Dorn aknowledged the Codex Astartes), well as Mix-Up stated, the Fortress of Eternity was a trap from the begining to the end, probably even the name was a trap, each step into the fortress was a kill zone, each corner, each passage, you name it... Dorn advanced even against the advice of his officers out of pure stubborness and the Imperial Fists began to amass critical numbers of casualties, to the point of having to use the corpses as cover. They finally reached the citadel in the center, just to find out that there was no Perturabo inside, just another killing zone. Well, as already said Ultramarines came and saved his sorry arse and those of the hapless Fists that were still alive, meanwhile the Iron Warriors made sure to deny the retrieval of the greater number possible of corpses. It was the offering of the geneseed of said corpses what gave Perturabo the God's approval and ascended him.
I don't know if the intention was to kill Dorn or humiliate him. But Perturabo and the Iron Warriors demonstrated once again that the Fists can't think on matching them. It was not only the mere sacrifice, but also the events Perturabo engineered what made him ascend. It wasn't just kills and sacrifices, it was also the terrible humiliation the Fists suffered what made the deal.
These events are ofter referred as "The Iron Cage incident"... another interesting fact is that GW never gave any official artwork about the current look of Perturabo as Daemon Prince, although he has been referred as "the ultimate obliterator" so you can have an idea...

7154225
I've heard that Obliterator comparison before. I hope he doesn't end up like their newest interpretation, which looks like metal plates and guns hammered onto an idiot Chaos Spawn. Seriously, they look like Syphilis Man.

Hopefully Perturabo will look more like either a Domitar-Ferrum, or take inspiration from the MUCH cooler Obliterator art.

7154182
I wonder if Perterabo's form is slightly less deamonic then what became of the rest of his brothers. Maybe a greater amount of his mass comes from technology, than mutation. Afterall, he didn't dedicate himself to one God, or the philosophy of the Undivided.

That would be appropiate, he already looked like an Strogg before daemonhood

7154307
I kind of feel sorry for Perturabo, after reading the hammer of olympia. He had everything he ever wanted however by his own hand lost it all.

7154459
Add to that his dreams of stablishing a proper greek style democracy, and suddendly happens that your father is the biggest dictator ever known

https://wh40k.lexicanum.com/wiki/The_Iron_Cage

This incident is the one that apparently led to his ascension. He apparently became a prince of chaos undivided, so technically is more of a freelancer than an employee.