Replace unit stats for only one map.

Activate the advanced map editor. Next, go to View => Game Database => New Window, then Resource => Select Tables => check the box next to the Creature resource. Open the desired map. In the window (the newly created one, with the "Creature" heading at the top), open the folders and search for Haven/Seraph. Double-click with the left mouse button. Select the Seraph resource. The second window will display its properties. At the very bottom, there will be the Abilities parameter. Right-click on it, click Add, and in the new line, select ABILITY_LIFE_DRAIN. Save and close the map.