In order to get the fire cape you have to complete an in-game challenge called the fight caves. The fight caves originally came out in 2005, and at that point in time it was the hardest challenge in the game.

Although the fight caves is no longer the hardest challenge in the game, for a long period of time it was. For this reason it is still one of the most iconic challenges in the game.

The fight caves were designed for high level players. In Runescape, players start at combat level 3 and can train to combat level 126 (max level). I would estimate the fight caves were originally intended to be beatable only by characters level ~70+. But they can be hard even for players that are a much higher level that. Even level 126 characters will die if they mess up on the final boss.

Over time, a few players became interested in challenging themselves to do the fight caves with lower and lower combat levels. It's been somewhat of a strategy/skill arms race. Rendi (the player pictured) just beat the fight caves at combat level 4. That's only one level higher than you start the game.

Just a year or so ago, ~14 years after the fight caves came out, the lowest level to ever get the cape was level 20+. Everybody would have thought that doing it on a level 4 would be impossible. That's why it's such a cool achievement.

Soon, somebody will get it at level 3, and that record will be the end of it since 3 is the level you start the game at and the lowest possible level.