The Rebeldes
The Hollow King has returned to Hueco Mundo, and he isn't pleased with how his homeland has turned out in the past century. There are rebellions in every corner of Hueco Mundo now. Some opposing the Hollow King's rule and Aizen's as well. Of all these rebellions are a group of arrancar and hollow known as the Rebeldes. These beings oppose both the rule of Ganga and Aizen and they fight for the greater good of Hueco Mundo. Having the misfortune of being born as hollow, these creatures are against everything that they are and will stop at nothing to overthrow the rule of evil's grasp around Hueco Mundo. They pose a threat to both the Hollow King's Espada and Aizen's Privaron. It can also be noted that not all of these guys are as good as their partners. Like most hollow, some are in it for a munch greater goal.
Battle of Las Noches
With Aizen still planning his return, the Hollow King has reassembled his army of Espada at his side for a full onslaught invasion on Las Noches, the headquarters of Aizen and his former espada known as the Privaron. Aizen is aware of this upcoming invasion and has made preparations with a full defense of Privaron at his hand. Each of them are Aizen's right hand generals and are basically in the frontlines in battle. On his left side are Aizen's Exquias. The equivalent of the Sereitei's Stealth Forces. These arrancar are basically the at the front of the line in battle.
Being the equivalent of the Soul King, the Hollow King known as Ganga has prepared forces of his own to invade Lad Noches as you already know. At his side, he's bringing together his newly formed and evolved Espada who have utmost loyalty to him. Like Aizen's Privaron, each of these Espada are considered high generals within his army however, with them the Espada bring their Fraccion. The Fraccion are chosen from numero's and serve their superior Espada munch like a Lieutenant does his Captain in the Soul Society.