Here is the full list of all 12 Blood Knights achievements.
3 vampiric decisions made
Roya stayed unharmed collecting the vampire blood
Kill 10 enemies by pushing them into a pit
Blood Seal returned
3 human decisions made
Kill your co-op partner with an explosive barrel twice
Kill 10 enemies with an explosive barrel
Revive your co-op partner 10 times
Suck 1000 litres of blood
Play through any act with a co-op partner
Kill one of your former friends
All Blood Coins collected