How does The Fire-Feathered Serpent World Change work?
To sum it up, killing 3,000 Seacrest Serpents spawns Renegade Quaras, however when 1,000 of all Renegades are killed they disappear and Seacrest Serpents spawn again. I'd like to give you additional details about this world change that is helpful. To know what stage this world change is on it's important you ask Guide NPCs "Hi -> Change -> Sea Serpent" As kill statistics on Tibia's website aren't up-to-date.
If under 1,000 Seacrest Serpents have been killed on your server the Guide NPCs will say "The Fire-Feathered Serpent is fast asleep."
When 1,000 Seacrest Serpents have been killed the message "Sleeping beneath the Oramond sea, the Fire-Feathered Serpent twitches and dreams of calling its children back" will appear in the server log or game window for all players. Also, the Guide NPC will say "The Fire-Feathered Serpent dreams and the earth is bleeding lava." When you ask them about this world change.
After an additional 2,000 Seacrest Serpents (or a total of 3,000 Seacrest Serpents) have been killed another message will appear in the server log or game window for all players, "The Fire-Feathered Serpent has awoken! Renegade Quara attack the sunken lands of Oramond!" Also, the Guide NPC will say "The Fire-Feathered Serpent is awake. Renegade Quara control the sunken regions of Oramond. "
This means Renegade Quara's will replace the Seacrest Serpents until your world has killed 1,000 of them then they will disappear and Seacrest Serpents will reappear with the message, "The Fire-Feathered Serpent goes back to sleep, beseeched by the blood of the Renegade Quara." I don't think any missions cancel the Renegades out, or at least I can confirm that completing the "Snake Charmer" achievement or fully completing the orb missions of Twenty Miles Beneath the Sea Quest doesn't make the renegades disappear in my experience. Also, according to Tibia Fandom when this world change is active you cannot complete Sub-Mission 1: Souls Lost At Sea.
Why is this world change important?
You get to complete "The Orb of Dark, Life, and Light" of the Twenty Miles Beneath the Sea Quest. Which can give you items you cannot get anywhere else-
I recommend you read about the orbs in this TibiaQA question Is it possible to obtain more than one orbs from Twenty Miles Beneath the Sea Quest? also TibiaWiki's Guide on the Twenty Miles Beneath the Sea Quest for more details on this incase you want to keep some orbs I highly urge you to read about them first.
Also, you can receive, 2 additional achievements Mind the Step! and "Snake Charmer" from completing "The Orb of Dark, Life, and Light" missions of the Twenty Miles Beneath the Sea Quest.
The Renegade Quara's drop great loot but also are on the bestiary and once you complete the Renegade Quara Constrictor, Hydromancer, Mantassin, Pincher, and Predator entries give you a total of 125 charm points. Each of these creatures requires 1,000 kills and provides 25 charm points each. You cannot see these creatures anywhere else so they are rare to have on the bestiary. It took me maybe 10+ of these world changes to complete my Bestiary for them. As you know, once a total of all Renegades are killed the world change isn't active. Also, all players are informed when this world change is active so it's hard to complete the bestiary without any interruptions. It's important to note these Renegades give easy profit so it certainly is fun to kill them, at least for me anyway.
Here's an image I created of the raid messages as proof of me testing this answer for hours and hours solo style-