A straight answer would be "yes" but it may not be as simple as it seems.
As you may know, demons are summoned to Gielinor by powerful mages - or, if they're powerful enough, they can build portals and travel between infernus and Gielinor. When they're hurt enough, their bodies explode to dust, but most of the times they're not killed completely. From what I've learned, it depends on how much of the demon was summoned to Gielinor. If the demon's essence was not completely summoned, the rest of it remains on infernus.
We also know that demons are known to get hurt when trespassing the barrier between infernus and Gielinor. And this is the interesting part. Demon Generals often come to Gielinor to spread terror, but they're not summoned, they build their own portals and come on their own. So I assume all their essence travels with them. This might mean that when they're killed, they're not returned to Infernus.
Now let's look at Thammaron's case. He did more or less what demon flash mobs do. For those who don't know - you learn this on The Golem quest - back in the god wars Thammaron was a zamorakian general. His goal was to destroy Uzer and he was tricky enough to engineer a nearly flawless plan, by building a portal below the temple of the city, that way his army attacked the impregnable city. They overwhelmed the soldiers and were nearly victorious, if it wasn't for the golem army. The golems wounded Thammaron so much that he had to return to his throne room, on infernus. We have no idea if he was further weakened - or strengthened - by returning home. He later slowly died from his wounds. This is the weird part. I know The Golem was one of the first quests of Runescape so the developers didn't know what they know now (I think it needs a revision). I think Thammaron was far too strong to die just like that. Maybe he consumed another demon (like flash mobs do) to heal himself a bit, and the skeleton in his place is just one of his underlings?
10-Jun-2017 19:06:26