Usually through some contrived magical bullshit that can ensnare them. Look at BG3. All of the party members should be ludicrously powerful from the experience they have. Karlach was fighting for a leader of hell, Wyll was fighting through hell trying to find her… they should be way higher level than 1 but the tadpole sapped them of their energies and kept them stuck in their form. Really you can come up with anything even remotely similar to that for your game. Doesn’t need to be a tadpole. Could be a curse or that you pissed off a god or a dozen other things. That’s the great thing with DnD. If you came think of even a slightly sensible way to do something then you can make it work. Hell even it isn’t sensible you can make it work.
Usually through some contrived magical bullshit that can ensnare them. Look at BG3. All of the party members should be ludicrously powerful from the experience they have. Karlach was fighting for a leader of hell, Wyll was fighting through hell trying to find her… they should be way higher level than 1 but the tadpole sapped them of their energies and kept them stuck in their form. Really you can come up with anything even remotely similar to that for your game. Doesn’t need to be a tadpole. Could be a curse or that you pissed off a god or a dozen other things. That’s the great thing with DnD. If you came think of even a slightly sensible way to do something then you can make it work. Hell even it isn’t sensible you can make it work.