Me and the DM are fairly new to D&D so we're still trying to figure out the rules. DM says that spells with a duration of "1 reaction" (ie. Shield) work as described below.
I totally acknowledge that the DM could make this a house rule, but that's not where we are (he said he thought it was in the rules somewhere). I'm not able to find anything confirming his interpretation in the PHB, so I wanted to get the internet's interpretation before asking our DM about it again.
- On my turn, "pre-cast" Shield as my action.
- Before my next turn, I'm hit by an attack.
- After learning the attack hits, I use the reaction to get +5 AC.
- DM compares to his attack roll and determines whether the attack hits retroactively.
- +5 AC lasts until the start of my next turn.
DM claims step 1 is necessary and consumes a spell slot. I claim that step 1 is not necessary and the spell slot is consumed after I use the reaction. Thoughts?
Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell
Components: V, S
Duration: 1 round
An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile.
Some spells can be cast as reactions. These spells take a fraction of a second to bring about and are cast in response to some event. If a spell can be cast as a reaction, the spell description tells you exactly when you can do so.