The third benefit of the War Caster feat (PHB, p. 170) says:
When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature rather than making an opportunity attack.
When casting a spell as a reaction through the War Caster feat, nothing is mentioned regarding the use of slots. So is this a free spell, or does it fall through back to the standard spellcasting rules, which would require the usual expenditure of a slot?
Cantrips also classify as spells, but they are considered level 0 which do not expend spell slots.
So you can actually cast a Shocking Grasp for example, or any other cantrip you would like! If you use a level 1 or higher spell then yes, you do have to expend the appropriate spell slot.
The text also does not clarify if there are restrictions on the type of action the spell requires so you can use any kind, be it Reaction, Bonus Action, 1 Action and all those with or without concentration to maintain (like the Hold Person spell for example, which can be maintained for a minute with concentration).
Yes, it uses a slot. You cast a spell, so it uses normal spell casting rules. Nothing about the feat says the spell is free or otherwise creates an exception to change the rules surrounding how the spell is cast.
