Its pretty straight forward and simple.

Step 1 : Find a Ritual Alter. Best place imo to find an alter easily is the Hun Camp. Don't use the alter yet.
Step 2 : Reduce your HP to below 250. This will make sure that your HP goes down to 1 after using the alter.
Step 3 (slightly tricky part) : Find a 2nd alter in the same map or in the next map (next map is the best option in my experience, Aqueduct in particular) with only 1 HP.
Step 4 : Use the 2nd alter with 1 HP.

Do this and the achievement is yours :)
Happy hunting. Video guide below if interested.


Video Guide :