Lone Adventurer is an NPC located at The End island. It's designed to guide you through and to help you with a pretty good starting loadout that will help you with dragons and Zealots!

To start the quest line, you must simply reach Combat Level 12 and speak to the Lone Adventurer which is located at the entrance of the island just past the shop on your right.

Your first interaction is to tell you to collect Ender Armor pieces, if you continue to talk to you he'll suggest that you kill 5 Enderman - do so to acquire another quest where he'll offer you a void sword for 200,000 coins, of course if you think the sword you have is better you can deny this offer, if you think you make a mistake you can go back to Rusty and buy it.

The next quest you get is to achieve and wear the Ender set. So start killing Enderman on the top/middle layer going further down doesn't help or make a difference, keep doing this until you get all 4 pieces of armor and the Ender Necklace. You the need to go and mine the ender nodes, which we have a guide on finding here, they are shiny obsidian and endstone blocks keep doing this until you obtain the Ender Gauntlet. For the Ender Belt and Ender Cloak you'll get them from Obsidian Defender and Watchers respectively.

Once you've collected the set (and hit Combat 15), leave the island and come back (this helps if the Lone Adventurer is missing). Talk to the Lone Adventurer just before the Dragon Nest gate, he'll ask you to find his Dragon Shortbow. Run around killing Watchers until it drops (sometimes you may need to leave and join the island and talk to the adventurer again). Hand over the Dragon Shortbow and then go visit the pearl dealer and he'll offer the Dragon Shortbow to you once you hit Combat 18.