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)
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?