I want trap my party in the realm of dreams without them knowing, but can’t put half-elf to sleep. Any ideas? how about send them when at 0 hp? Would make reviving important, especially if one round in realm is like 5 or even 10 in real life
— Mike Mearls (@mikemearls) October 30, 2017
Half-Elves have resistance to magical sleep, but they do still sleep naturally every night, unlike full Elves. If all you need is a way to get them asleep in order for them to become trapped, you’ve already got one.
If there’s a limitation to how they get trapped – for example, if they would normally wake up after sleeping for awhile, but you want to keep them trapped by magically extending their sleep / preventing them from waking up, there’s a fix for that too.
Half-Elves only have resistance to ~magical~ sleep. There’s nothing stopping you from keeping them unconscious via non-magical means. For example, a poison of some sort could work nicely.
Maybe the party interacts with a kindly seeming NPC who offers them hospitality in the form of good food and drink, but it is laced with a subtle sedative, and the party goes off to bed full and happy, and very sleepy, to the point that even if they set up a night watch, the person on guard still falls asleep. Then they’re kept asleep with regular doses of more sedative.