Don't use it or play it. IMO, it isn't worth the slot when there are more effective cards.
Reason (if you need them):
1. Potter's Field needs other cards to even work and I believe (aka without looking it up) they are spread out through different brigades.
2. It is an outdated card.
3. There are more effective cards to return you your heroes.
a. Midwives for OT characters
b. Chariot of Fire
c. Any heal card in your hand
d. David's Harp
The list goes on. Sorry that you pulled a pretty useless card but that comes with the territory of booster packs.
