A Phoenix quill is the secondary ingredient in creating Phoenix and Reborn phoenix pouches. The five only known ways to get these so far (except by buying from other players or the Grand Exchange) are:
10 phoenix quills can be used to skip directly to the fight against the Phoenix, avoiding all the Reborn creatures in the previous caves of the Phoenix Lair. As you get five from successfully slaying the phoenix, this comes to a net loss of five quills.
It takes roughly 2 minutes to go through the Phoenix Lair which means that paying 10 phoenix quills breaks even if you can otherwise make 1,210,800 coins per hour.