The reward for your sacrifice is the ability to
Resurrect yourself. For each level you reach on your way, you will be able to do so once a week. Thus as a Knight of Sacrifice (level 3) you can resurrect yourself 3 times a week.
After resurrecting yourself you will find all your belongings back in your backpack, except for any items that were looted in the time it took you to resurrect. The items will not be returned if you crossed a server line (e.g. walked out of a dungeon) before resurrection.
To use the virtue you'll click on the virtue icon on your paperdoll(it is over your head) then double click on the Sacrifice icon(teardrop).
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"Farming" is simply killing monsters in dungeons that will give the most fame. Demons, Liches, Lich Lords, Ancient Liches, Balrons, etc. are some of the more popular monsters farmed for fame.