So the entry for Erinyes has the following:
Hellish Weapons. The erinyes's weapon attacks are magical and deal an extra 13 (3d8) poison damage on a hit (included in the attacks).
Then we have the following entries for weapon attacks:
Longsword. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) slashing damage, or 9 (1d10 + 4) slashing damage if used with two hands, plus 13 (3d8) poison damage.
Longbow. Ranged Weapon Attack: +7 to hit, range 150/600 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 13 (3d8) poison damage, and the target must succeed on a DC 14 Constitution saving throw or be poisoned. The poison lasts until it is removed by the lesser restoration spell or similar magic.
In a recent battle an Erinyes was disarmed via the battle master ability and lost possession of the longsword. Would this longsword retain its properties if used by a player character?
At first I was leaning towards no, as the description of 'Hellish Weapons' says that the attacks are magical and deal extra damage, not that the weapons themselves are magical. But then I noticed that the longbow had a different poison effect on its attacks, which would lead to thinking that the weapon imparted the poison effect.
The closest comparable weapon is the Flametongue, which requires attunement and deals an extra 2D6 fire damage (rare). Would this weapon be OP in the hands of characters? Would this be comparable to any other magic swords at the very rare or legendary tier?