Eowyn + Stand and Fight is a way to smooth resources or “splash” an off-sphere card or cards (this means include cards that don’t match any of your heroes) in the Core Set. Eowyn’s ability let’s any player discard any card from your hand for +1 willpower. If a non-neutral ally is discarded, it can be targeted by Stand and Fight and put into play. They don’t have to be Spirit, but you have to use Spirit resources to pay for them.
For example, you can include ally Beorn in Leadership & Spirit decklist from the Revised Core rulebook. Discard him to give Eowyn +1 willpower during the quest phase. Later during the combat phase, play Stand and Fight for 6 Spirit resources to put Beorn into play ready to defend or attack. There is another example that can be a great tactical play to counter the 5 threat 1 quest point location Brown Lands. In the action window after staging, discard Snowbourn Scout to Eowyn’s ability, then play Stand and Fight targeting Snowbourn Scout, and place a progress on Brown Lands to explore it.
A common mistake made with this combo is trying to target Core Gandalf. Stand and Fight says, “the chosen ally can belong to any sphere.” This phrasing has confused many players as it seemed to mean Stand and Fight could target any ally, but it is actually a restriction. The first FAQ ruled that neutral cards do not belong to a sphere making neutral allies ineligible targets.
Another important rules distinction is that Stand and Fight puts the ally into play but it is not considering “playing” the ally. The combo then is very helpful in quests like Escape from Dol Guldur that limit the number of allies you can play per round. It also means that cards like Erebor Hammersmith that have effects that trigger when you play the card, putting them into play with Stand and Fight doesn’t trigger the ability.