Super mobs (SM for short) are special mobs that are released by Immortals. The mobs are typically released at an admin's discretion (usually when a lot of people are on that can fight and get the dropped item from the mob) but a new mechanic allows players a chance to summon them as well. You must be below a certain powerlevel in order to get the drop from a Super Mob.
For the full list of Super Mobs, powerlevels, and drops, see "Help Supermob" ingame.