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