Old Man

The Old Man is an NPC found outside the Dungeon. He "guards" the entrance, and the Dungeon cannot be fully explored until he is gone. When spoken to at day, he will tell you to return at night. When spoken to at night, he will give the player the option of fighting his master, Skeletron. He will deem players with less than 300 life (15 hearts) or 10 defense as not worthy, but still allows them to summon Skeletron.

When entering the dungeon without killing Skeletron, Dungeon Guardians will spawn and instantly kill the player.

The Old Man can be hurt with lava (he only takes one damage per second), or traps (natural dart trap tested only) which causes normal damage and poison effects. Killing the old man is pointless, because his death does not eliminate the Dungeon Guardians that will appear inside the dungeon. If he is slain this way, or the player fails to defeat Skeletron, the Old Man will return to the Dungeon during the next day.

When Skeletron is killed, the Old Man is freed from his curse and will arrive at your house as the Clothier in addition to allowing access to the Dungeon. Thus, you can only fight Skeletron once per world as if you kill the Clothier a new one will spawn if a house is still available, not a new Old Man.

Quotes
Only during day: Only when you have 300 or less max health or 10 or less defense: Only when you have more than 300 max health and more than 10 defense:
 * "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."
 * "You are far to 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."
 * "Stranger, so 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."