So a quick summary of Gulgrim's (probably) final attack, so as to avoid confusion.
Gulgrim has finally unleashed just about every last drop of WAAAGH!!! energy that he's been building up, both during the fight that he's been participating in, and from his long death march. Prior to unleashing it, it was overflowing so much that he was losing a bit of control with it, which is why his telepathic message was almost painfully forceful to receive, to the point that he couldn't even control who heard it, even Soul Reaver got it, and why his regular actions were so over-exaggerated.
Now that he's unleashed it, it intially manifests as a directionless green hurricane around him that is dangerous for anyone to be in. Anyone close to Soul Reaver and Gulgrim is at risk of being basically zapped, bashed, and slashed by razor winds of raw power. After a few moments of subjecting the area around himself to this, he begins to focus his energy into a point, and it goes from a hurricane to a hammer, which he begins to attempt to smash Soul Reaver into the ground with.
During this time period, while there is still a lot of dangerous phenomena occurring around Gulgrim; rocks flying, lightning striking, etc., it's not really dangerous to anyone of our heroes' power level, and mostly for special effects. Gulgrim himself is motionless and unresponsive. Attacks towards him will almost certainly strike as he is currently unable to act while channeling so much raw energy into a focused attack like the one he is attempting to use on Soul Reaver.
The raw force itself is pure WAAAGH!!! energy, and is attempting to slam Soul Reaver into the ground repeatedly under its force. The primary attempt at first is to incapacitate him, but if nobody has anything to do to try one more attempt to save Soul Reaver, hence gulgrim's 'One chance' speech, he will proceed to attempt to kill him.
If anyone has any questions or issues, feel free to PM me about my post.