Nymphs are very rare, and pretty much only obtainable from the oracle. They are hard to get, which is why they are considered so awesome. Also, many of them have abilities that are awesome when able to use multiple times. For instance, getting the gravity nymph will let you use black hole on the other person (eat some of their quanta and gain health) multiple times a turn, which can be very devastating. Nymphs are seldom used because of their scarcity, but they can make a good deck better sometimes.
