Zaen had killed almost two hundred people that night. All those kills gave him enough EXP to get to level 51. Thus he had overcome the barrier that was stopping him from using the crystals he had earned after killing the boss level monsters.
Everyone thought he would escape while his invisibility lasted. But he wasn't trying to do anything of that sort. He simply rushed to a safe place so that he could use the crystals and learn the abilities hidden within the crystals.
Not to mention his both his HP and mana had also shot up by a ridiculous amount. He now had 6450 HP and his mana was 1700 which was enough for him to use the Silent kill zone spell that he got after using the crystal dropped by the Depraved Ghoul.