Mockingbird's message is revealed in Thunderbolts annual 2000. After Daimon Hellstrom used the Thunderbolts to rescue Patsy Walker from hell, Hellstrom explains that he was just carrying out Mockingbird's wish. The wish apparently was "help Daimon--Patsy Walker is going to be used by Mephisto as a pawn to kill him!". Mockingbird apparently appeared in Hellstrom's throne room after disappearing from the Avengers and completed what she was going to say. Hellstrom says he couldn't invade Mephito's realm to rescue Patsy without causing a war so he had to do exactly what Mockingbird's message said.
The revelation of the message didn't seem real. Hellstrom could have just made up everything that Mockingbird completed the message in front of him before disappearing. He already manipulated Hawkeye and the Thunderbolts with his current explanation it's not that hard to conceive that the whole scenario was just made up by him and that no one really knows what the message was supposed to be. Rather than having Patsy Walker return from the dead it would have been have more worthwhile to the Thunderbolts to have Mockingbird return. Mockingbird's death was pointless and her return could have at least provided some interesting interaction for awhile. Maybe with upcoming Avengers annual with Patsy walker more will be revealed about the message but I'm not expecting it too.
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