A recent D&D Next blog article raised the topic of energy drain. Traditionally, energy drain causes a loss of levels, which is terrifying to players because it invalidates real-world effort (which is of course the point; the only way to really make undead scary is to harrow the player rather than the character). However, level drain is incredibly unpopular with the mainstream of players, because most people don’t understand that some monsters are really obstacles, not foes.
That being said, level drain is unlikely to be part of 5E, even as an option. Here is what they are proposing as a replacement for losing levels:
Level drain is a bit trickier, but we want to try modeling the drop in potency with a reduction of your maximum hit points (something that can last for a long time, but could be removed via spells or sufficient rest).
This actually seems like a reasonable method, assuming that the maximum hit points do not recover until the next level is gained (unfortunately, “sufficient rest” likely means the next extended rest, which totally neuters the threat). If it were to last until the next level it makes characters more fragile in a way that does not require a major overhaul of the character sheet. It also has more direct impact than ability score damage.
Further, what if the effect was framed as a curse that could be lifted with appropriate labor? Not a curse that could be lifted with remove curse, or the penalty just becomes a money sink. So players have a way to get back to full strength (at diegetic cost). Additionally, the effect could also automatically go away upon gaining a level. Thus the effect becomes similar to a level drain by default, but can also be the jumping off point for side quests (particularly ambitious referees could incorporate some aspect of the spirit’s past existence, such as a proper burial or restitution to victims).