I bought the base game for Eldritch horror recently and I have one point that I needed clarification on. I have two monsters on a particular space and I cast the Wither spell for my first encounter successfully gaining three strength. I did not defeat the monster and for my next combat turn, would I still have the three extra strength or would I have to pass a lore test again? Also, on the next turn, would I have to cast the spell again for its effect or would the effects stay since I am battling the same monster. Kindly advice.
From the base rule book, page 9:
If an investigator is on a space containing one or more Monsters, he must resolve a single Combat Encounter against each Monster on that space, one at a time, in the order of his choice. Combat Encounters are described in detail on page 14.
After an investigator resolves all Combat Encounters, if there are no Monsters in his space, he may resolve another encounter.
The card effect only lasts as long as the combat encounter, and the combat encounter lasts only for fighting a singular monster.
The Wither Card states:
When resolving a Combat Encounter, you may test Lore. If you pass, gain +3 Strength during that encounter.
Hence, the combat encounter is the process in fighting a singular monster and you can have multiple combat encounters in a singular encounter phase. Wither only lasts for a singular combat encounter (single monster) and gives +3 strength for it.
You would need to cast the spell again in the next combat encounter if you wanted it's effects again.
Make sure you flip the card each time you cast the spell to resolve the effect on the back.