Getting your Stardust battle pet (beating the Celestial Invitation)

This is my entry for #archdruidcontest "most prized gaming possession" contest.
I recently completed this milestone in World of Warcraft, and like my zookeeper title it has been years in the making.

For those who don't know "pet battling" is a mini game within the World of Warcraft very much like Pokemon.
You collect pets from many sources within the game: world drops, boss drops, capturing within a pet battle, vendors, crafting, and achievements.
Each one of your pets can be renamed, leveled from 1-25, and given a slot on your pet battling team.
The rarest pets command some of the highest gold prices in the game, and some of them, like Stardust are one of a kind, and show other pet collectors just how dedicated you are.

This journey begins with the raid bosses of the Wrath of the Lich King expansion: collecting 16 unique pets which drop from the raid bosses of that expansion.
https://www.wowhead.com/achievement=11320/raiding-with-leashes-iv-wrath-of-the-lick-king
Each of the pets has an around 1-10% drop rate, so be ready to run these raids quite a few times.
Of course, each raid has a "lockout" that makes it so they can only give you loot once a week, so also be prepared for this to take months to years to complete.

Once you win your final pet in the list (for me it was the wicked soul which drops from 25 man heroic Lich King) you will get a celestial invitation in the mail to battle Algalon in a pet battle.
For those not familiar with World of Warcraft lore, Algalon is a celestial analyst of sorts, working for the creators of Azeroth, who for a brief moment was not sure whether Azeroth deserved a continued existence or whether it should be "reprocessed."

For me, the Algalon fight, in the Ulduar raid, is the most visually stunning boss fight in the game, and I'm glad that I still need a mount from Ulduar, since getting to see it over and over is definitely a highlight to my gaming time.
However, after reconsidering and deciding that we weren't so bad after all (since our love of collecting pets has made us relatable to this celestial collector) Algalon has put together his crack team of celestial pets, and invited you to battle him!
I pulled up with my beefy arsenal of level 25 pets on my recently won astral cloud serpent mount (1% drop chance from Mogu'shan Vaults) and expected to have this encounter knocked out fairly quickly.
3 days later................

So, I'd recommend, regardless of what team composition you choose: come stocked with lots of pet bandages, and even more patience.
Algalon's team hits like trucks, and basically two lucky crits from his pets will make the match unwinnable.
The team I used was 1) Zandalari Kneebiter 2) Murkidan 3) Emerald Protowhelp
Open with the Kneebiter's black claw and hunting party - he will die, so bring in Murkidan and finish off Comet.
Use Murkidan's nether gate to forcibly swap in Constellatius.
Constellatius will cast Apocalypse.
Swap the Emerald Protowhelp in and last the next 12 rounds by self-healing. Don't miss a heal.
Swap Murkidan back in to die on the 12th round, and when Apocalypse kills Constellatius as well, that will leave a 1v1 of the Protowhelp vs Cosmos.
This is where you just have to hope that Cosmos does not get lucky crits and that you can down it in time before your health gets ground down, even spamming both heal abilities nonstop.
PSA: this isn't the ONLY way to beat this fight - it is just what worked for me.

But, when you beat it, what a satisfying feeling!
And best of all, you get possibly the most unique pet in the game.
As if being a rabbit made out of stardust wasn't enough, this pet will frequently turn into a comet, fly up into the sky, zoom around, and then come plunging back to earth in a shower of stars.
How cool is that?

So, I think this makes it pretty clear why this is now my most valued gaming possession.
And you should go get yours as well!
I think this one has a bit more lasting uniqueness due to some of the rare mounts, which you might find out, just aren't that rare when there are so many other determined collectors in the game.

(Ok, so the astral cloud serpent mount isn't super-rare since there have been so many years it has been farmable but it sure goes well with stardust)