Dennis the Lame
Following the Faith Militant's defeats in the battles at Stonebridge and the Great Fork of the Blackwater the previous year, new leaders arose across the Seven Kingdoms in 43 AC, one of which was Dennis the Lame. He and his followers were hardly distinguishable from outlaws, ambushing royal forces then disappearing into the greenwood. Smallfolk gave them aid wherever they went, while the knights and men-at-arms who were sent out to bring them to justice often disappeared.
The exact region that Dennis the Lame and his followers were active in hasn't been stated. Dennis was still alive at the time of Maegor's death, but his ultimate fate has not been revealed.