It might have a good lore justification, but I think that a constant health drain just does not fit the exploration and talking-based gameplay of Original Sin 2 at all.
A different way to represent Source Addiction would be better. Hmmm... How about this, just a quick idea off the top of my head:
Source Addiction (Passive): Undead get a +1 Bonus to all attributes when their Source Points are full. When their source points are not full, Undead slowly become more vulnerable to damage over time, up to a maximum of -50%. The fewer Source Points they have, the faster their resistances tick down. They immediately lose this penalty upon acquiring a Source Point. After losing a Source Point, it takes 30 rounds/180 seconds/3 minutes before the damage vulnerability starts to take hold. (That's STARTS - so that's three minutes of normal play before the withdrawl kicks in.)
Thus, the undead are encouraged to keep Source Points, and they have a harder choice about when it's worthwhile to spend a precious Source Point, but the effects are not the same. It's not +20% which drains back down to normal, cancelling itself out.
