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. Thus, Skeletron can only be fought once per world, as if you kill the Clothier, a new Clothier will spawn (if a house is still available), not a new Old Man.

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."
 * "You pathetic fool. You cannot hope to face my master as you are now."
 * "I hope you have like six friends standing around behind you."
 * "Please, no, stranger. You'll only get yourself killed."

Only when you have more than 300 max health and more than 10 defense:
 * "Stranger, do you possess the strength to defeat my master?"
 * "You just might be strong enough to free me from my curse..."
 * "Please! Battle my captor and free me! I beg you!"
 * "Defeat my master, and I will grant you passage into the Dungeon."

Trivia

 * The Clothier will spawn only after Skeletron is defeated. Looking very alike, it's easy to assume that the Clothier is the same character as The Old Man. This is further proven by the Clothier himself, who will sometimes say: "Thanks again for freeing me from my curse. Felt like something jumped up and bit me"