How do the War Caster feat and the Enemies Abound spell interact when an opportunity attack is provoked?

Question

The enemies abound spell states:

[...] If an enemy provokes an opportunity attack from the affected creature, the creature must make that attack if it is able to [...]

- Xanathar's Guide to Everything (page 155)

And the War Caster feat states:

[...] 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 [...]

- Player's Handbook (page 170)

Note that a spell cast using War Caster is not considered an opportunity attack

I'm now unsure what happens when somebody is under the enemies abound spell but also has the War Caster feat and an opportunity attack is provoked. Does War Caster allow them to cast a spell or does the phrasing of enemies abound (especially "that attack") require them to make the opportunity attack?

Answer 1

Interpreting strictly by the rules as written, I'd say you have to make the normal opportunity attack rather than casting the spell: War Caster is worded as being instead of the attack, as you noted, so if you make that choice you're not making the attack.

If instead of rules as written, you want to know how I'd actually rule this situation as a DM, I'd give a different answer. I'd allow people to use the spell substitution for War Caster, as long as their choice of spell to cast was clearly hostile - Sacred Flame or Hold Person yes (even though neither actually involves making an attack), Guidance or Cure Wounds no. This option does rely on the group as a whole being willing to buy in on that sort of decision-making - if the players prefer an unambiguous ruling (for example to avoid edge cases where it's not clear if a spell is hostile in intent or not), or if there's significant disagreement on interpretation in play, I'd fall back to a strict read and only allow standard opportunity attacks.

Allowing spells does introduce a lot of complexity. The Hold Person example is pertinent, as it and similar concentration-based spells would allow a player to cast a spell and then immediately drop concentration, effectively avoiding the attack having any effect. If players are interested in exploiting that sort of edge case I'd definitely stick to the strict reading, as it's much clearer and easier to keep consistent under pressure.