Diablo IV is hosting an invitation-only event (in the United States) for players to donate blood and then confirm it on the Diablo Blood Harvest website in order to claim in-game items.
This Diablo IV event aligns with the current in-game content, Season of Blood. The blood donation will tally up the total amount of blood from all players. If the goal of 33%, 66%, or 100% is reached, more and more items will be unlocked. The highest-tier item to be unlocked at 100% is the Diablo Custom Liquid Cooling PC, which c ools down with actual human blood. The players are invited to participate and vie for the chance to win.
Source – Diablo
TLDR: Diablo IV is hosting a blood donation event where players can unlock in-game items by reaching donation goals. The ultimate reward at 100% is a Diablo-themed PC using real human blood for cooling. Players are invited to join and compete for the prize.