I found this sentence I wrote almost a year and a half ago:

Watching all the S&T geeks (like me) go nuts over Tren Krom's final line in Reign of Shadows chapter seven will be entertaining. 8D

Which suddenly made me decide to look up RoS chapter 7. Last few sentences:

"Where are we going?" asked Hafu. "And do I really want to know?"

 

Tren Krom pointed to the Coliseum. "There. I have a message for Mata Nui. It may mean the difference between life and death for everyone."

 

"Mata Nui?" asked Hafu, incredulous. "But Mata Nui isn't there. Makuta Teridax exiled him from the universe, maybe killed him. How are you going to get a message to him? And what could he do to help us now, anyway?"

 

Tren Krom looked at the Po-Matoran. A strange smile came to Lewa Nuva's mouth, the corners of it bent at an odd angle. "The answer to both those questions is the same ... you would be surprised, Hafu. Very surprised."

So, was it ever explained what it was?