62. River Merrow

The PCs are ambushed while trying to cross a fast flowing river by a group of Merrow. The players could be crossing via bridge/barge/raft depending on where you are dropping this encounter, regardless the Merrow attempt an ambush and use their harpoons to try and pull their victims into the river and drown them. Each round on initiative 20 the river's current carries creatures 10 feet downstream, any creature with a swim speed can choose whether they are carried downstream or stay in position. Whenever someone is pulled into the river one Merrow will try to grapple the target and another Merrow will use the Help action to maintain the grapple, they will drag the target below the water and then swim downstream together dragging the target.

For a less deadly encounter, have random NPCs crossing at the same time so that they can be targeted instead of the PCs, or rather then trying to outright drown the target they can just be taking them prisoner and bring them to their lair. Or even have a merchant caravan be part of the crossing and the Merrow target the goods, they try to harpoon and drag wagons, chests, etc... into the water which they can then loot at their convenience.