Farm the Lernean Hydra in Legendary

The optional Lernean Hydra spawn in the Athenian swamps, Act I, Legendary difficulty only.

Before you go

Unlock Legendary difficulty and reach the Athenian swamps.
Bring high poison resistance; fire and cold resistance also help, especially at range.
Carry healing supplies and a build able to handle a multi-attack boss.
Leave inventory space for high-level loot.

Enter the Athenian swamps on Legendary and follow the side route through the marsh rather than the main campaign path alone.
Search the out-of-the-way swamp area for the Hydra encounter.
Approach with poison defenses ready and avoid standing in breath attacks if fighting at range.
Engage the Hydra, using sustained damage and healing as needed.
Defeat it and collect the boss loot and any associated reward chest.
Return to town, sort gear, then restart the session to repeat the optional boss route.

Melee characters can reduce exposure to breath attacks by staying close, but still prepare for its poison bite.
At range, add fire and cold defense alongside poison resistance and watch for the boss changing targets.

Repeatable on Legendary after a session reset while the optional spawn is available. Loot varies; no particular legendary drop is guaranteed.

Looking for Hydra in Normal or Epic: this optional boss is reported only in Legendary.
Neglecting poison resistance: prepare that defense before attempting repeat clears.
Assuming the main swamp path automatically passes the boss: search the side area for the optional encounter.

Stop when you have the upgrades needed or the kill becomes inefficient; use a later boss route if it better matches your gear goal.

Sources & fact checks

Open a guide to find other locations and see what the item is used for.

Boss and chest runs

The optional Lernean Hydra appears only in Legendary difficulty and is a supported Act I loot target for characters seeking…