Hollow Knight: Silksong has a variety of NPCs that give you quests, which end up in pretty important rewards that can make your life that much easier when going up against the several intimidating bosses in the game. One such key upgrade to your weapon is tied to the flea NPCs.
Closing this quest gives you the 3rd Pale Oil (here are the locations for the first and second Pale Oil if you have not gotten to them already). The quest is the Ecstasy of the End, and here is how to go about getting and completing this.
How to Start the Ecstasy of the End Wish in Silksong

Complete the entirety of the Lost Fleas quest to move the Flea NPCs to the Putrified Ducts. Next, do all the steps to reach Act 3. Once you have Act 3: Abyss unlocked, visit the Bellhart Wishwall and pick up the Ecstasy of the End wish. Speak to Fleamaster Mooshka at Fleatopia to start the wish.
You will be taken to a newly decorated area where the rest of the fleas are celebrating the Festival of Fleas. Your goal now is to play all three mini games and get the highest score possible to reach the very top of the leaderboard.
Speak to Mooshka, Vog, and Kratt to start any of the mini-games. Let’s take a look at each game individually and how to get a high score.
How to Win the Flea Juggle Game

Your goal is to keep all the fleas afloat by hitting them with your weapon. These fleas will drop down on you randomly; you just have to keep hitting them to keep them airborne. Some of the smaller fleas will vanish after three hits, but the bigger fleas require more hits to leave the stage. Aim for 30+ fleas to win the game, check the flea counter on the bottom left corner to know where you stand.
How to Win Flea Dodge Game

The task here is simple: avoid as many fleas as you can to get the high score. This also includes avoiding any of the bigger fleas’ ranged attacks. Keep an eye on the platform below you; some of them don’t stay there forever and will collapse after a while. Using all of your dash and jump abilities here will prove useful. For this game, aim for 75+ fleas dodged, preferably reaching 80 before you hit the ground.
How to Win Flea Bounce Game

For the last mini game, try to stay afloat as long as possible by using the passing fleas as leverage. This will test your gliding and ascending skills without the help of walls or chains to hoist you up. The passing fleas get progressively faster, so try to aim for at least 40+ bounces.
After winning all three games, speak to the Fleamaster once more. To celebrate, he will summon a piñata. Hit it several times till it breaks, dropping the third Pale Oil, and completing the wish in Hollow Knight Silksong.