I don't think that this would be a very good addition as it currently stands. Allow me to explain why...
"The Sky turns 50 shades of Red" doesn't fit well in Dwarf Fortress' writing style. Not only that, but the fact that this would be via an announcement wouldn't make all that much sense. Being able to hear about it from news upon a map would be a very reasonable alternative, however, similarly to how you can view attacks on other settlements.
Also, using "Goblins" is too general; I think that using the civilization or the leader would be better. The name "The Great Destroyer" would also have to be substituted with a randomly generated alternative. Perhaps using multiple terms, while keeping the same pattern. (For example, The Great/Monstrous/Corrupted Destroyer/Spire/Peak.) You wouldn't really know that it is a gateway to the underworld, either.
However, "A beam of energy is coming from the top, tearing the fabric of the universe, this is a dark time for WORLD_NAME" doesn't fit DF's writing style as well, and the idea of the fabric of the universe tearing is a bit generic. I would substitute it for the following;
"From atop [TOWER_NAME], a beam of [Synonym of Corruption, Evil, Darkness, Etc] rises to the skies. Shadows turn darker, and clouds stir in the sky."
Then, the "Demons appearing in place of animals" point. I think that a better idea would be for hordes of demons to periodically appear high above the sky, with the amount of occurances coorelating with distance to the tower of darkness.
And, with raids, you should be able to enter the tower. However, the goal should not to find some sort of core and perform a sacrifice. Rather, the goal should be to seal off the entrance to the underworld that was created.
Also; There is a problem with this idea that I have as a whole. What would the goblins' motivation be? I only really have one idea; To create a greater opening to the underworld, and allow for more demons to exit the underworld. So, I'd imagine the new goal is to seal the gateway, and the reward being - not with the other dwarves becoming gods - but of the demon raids stopping. The remaining demons, lacking a proper portal to the underworld, I'd imagine, would become something similar to Night Trolls, periodically raiding sites in packs.
After the gate is sealed, the idea that all goblin civs would attack every other civ would be too extreme. The Tower would be destroyed or reappropriated, as the civ who conquered it would desire, and the goblin civs may try to take it back. They're pretty much 'at war' with all civs at the get-go anyhow, so that wouldn't change much.
I'd imagine, after the gate is sealed, the goblins would focus themselves almost completely upon the people who tried to take the Gate. '
So, in summary; This idea has potential for a possible event that could be done by large goblin civilizations. However, it is worded poorly, and quite a few things do not make sense. But the idea is still fairly decent on its own. So, good job.