Crimson Offering is a process introduced with the Rotten Blood Quest through which players who defeated the final boss Bakragore can obtain Sanguine Set equipment.
While hunting in the Rotten Blood areas under the influence of Bakragore's Taints, players can collect very rare drops that can be offered to the Altar in the Blood Vestibule:
- 5
Tainted Hearts - 5
Darklight Hearts - 50,000,000 gps
As a reward, they will get a 
Bag You Covet, which contains one piece of sanguine gear.
Although players can reach the Blood Vestibule at any time, only players who have defeated Bakragore at least once can make the offerings.