The Xebos Invitation is a random reward from any of Redemptionrsps's PvM. It is used to access Xebos Boss, the tyrannical emperor himself. Each invitation is a one-time use only. The invitations are automatically banked when they are received.
NOTE: This Invitation allows you unlock the Karma skill as well, in order to do that you will have to kill Xebos and his minions (who protect mage). You can follow the Karma Skill Guide for more information on the Xebo's fight for the Karma skill unlock.
At the moment, the price of the Xebos Invitation is roughly: 1.2M-3M Bronze Coins