Old Man

The Old Man is an NPC whose primary function is to summon Skeletron (the hardest pre-Hardmode Boss before the Wall of Flesh). He can be found at the Dungeon's entrance. When spoken to at night, the Old Man will give the player the option to "curse", which summons Skeletron. The Dungeon cannot be fully explored until Skeletron is defeated.

When spoken to during the day, the Old Man will tell you to return at night. He will also judge players with less than 300 life (15 hearts) or less than 10 defense as unworthy, but still allows them to summon Skeletron.

The Dungeon can be entered without killing Skeletron, but traveling deeper than ground level within it (below zero depth) will spawn Dungeon Guardians that will instantly kill the player.

When Skeletron is killed, the Old Man is freed from his curse and will arrive as the Clothier (as long as a vacant House is available), in addition to allowing access to the Dungeon.

Quotes
Only during day:
 * "Come back at night if you wish to enter."
 * "I cannot let you enter until you free me of my curse."
 * "My master cannot be summoned under the light of day."

Only when you have 300 or less max health or 10 or less defense:
 * "You are far too weak to defeat my curse. Come back when you aren't so worthless."

Trivia

 * In the mobile version, the old man will never believe that you can free him of his curse, due to the fact that the max health is 200.
 * The Old Man is the only human NPC besides the Guide who doesn't sell things, and doesn't need a house.