Assuming a D&D 5e game, I load Kobold Fight Club and click until I find monsters I can build a little story around.
A while back (including enemies from Tome of Beasts) I got Spawn of Akyishigal and Giant Ants, and after a few overland battles they found a beleaguered anthill.
By the next session I had my dungeon made and some lore surrounding it.
The giant anthill had carved its way into an ancient tomb of an orcish warlord who had managed to seal the Demon Lord of Cockroaches with her in an attempt at everlasting life. The actions the players take can result in her rising as a Mummy Lord or in Akyishigal being freed.
All from going "Hey, these enemies work well together."