A wild card to get you what you need when you need it. Often it is better to keep Onyx Tome if you are not sure what to return. While the extra magic can seem a steep cost in addition to the 2 wounds, those 2 wounds can work in your favor, allowing you to damage a unit that you were going to return to hand anyway and getting 2 boosts in the process. The versatility and + 2 boosts on Selundar are often worth its steep cost. Remember it can be used also to return champions to hand.