Siren's have been suggested before, but perhaps not with this mechanic for them.
First, locations: have siren's spawn on…
1) a "siren reef" (at roughly 1/2 the frequency as Troll caves, and maybe 2/3 the size of a troll cave) on the edges of ocean biome (maximum of 3 sirens spawned on a reef at any time)
2) on smaller "siren rocks" in the shallows (minimum water depth?) of other biomes (black forest and up. max 1 siren per "siren rock")
3) and as one-time spawns on Leviathans (max 3).
Sirens would possess no physical attack, but would periodically "wail" (like wolves periodically howl), which would do stamina damage at first, and then health damage if stamina is at zero at the time of attack.
Range limit on wail: roughly 15 meters maybe, or at least "somewhere between a Greydwarf's rock-throw and a Fuling Shaman's fireball toss".
The stamina drain could be offset by stamina meads (including 'tasty').
Also, sirens would not chase the player (thematically: sirens try to drown victims by luring them to their deaths on rocks, boat sinks, sailor has to swim but can't because no stamina), allowing them to move 'out of range' of the 'wail' to recover or escape (especially in a boat).
Drops: 'siren meat' which could be cooked straight like other meats, but would uniquely (among straight cooked meats) be a stamina focused food instead of a health focused food. (based on myths of mermaid meat granting immortality. it's a stretch I guess, but think: immortality => life => energy => stamina). Oh and maybe also 2 serpent scales as a bonus (from 'bra'? lol, ok no, sorry maybe not :P). Maybe pearls? (thinking of the mention of clothing from the recent dev blog. could be used for jewelry or sewn directly into clothing or whatever)
Source: https://www.reddit.com/r/valheim/comments/10e0rnq/ocean_enemy_idea_siren_not_a_new_idea_i_know_but/