Orochimaru
Original Orochimaru
The original Orochimaru is the basic enemy during a long time in the original Naruto series.
He is basically the arch-enemy Naruto has, before Sasuke's actions.
Being depicted as almost invincible, Orochimaru makes a good template for what is normally in Lotgd the Final Boss, the "Dragon".
Orochimaru in SL
In SL Orochimaru is the Final Boss you need to defeat in order to achieve a new rank.
After defeating him, you will be reborn, stripped of all gold, weapons and some other possessions, starting again at Level 1, but with 1 more Orochimaru Kill and possibly a new Orochimaru Kill Title as well.