Happy 12th birthday to the original Pixel Dungeon, first released on December 4th 2012!
Unfortunately it’s a bit hard to find the original Android versions these days, but if you’re okay with APK downloads, you can find an archive of old versions here: https://github.com/00-Evan/pixel-dungeon-gradle/releases/tag/archive
If you’re more interested in desktop platforms, the game is free on itch or paid on steam:
https://watabou.itch.io/pixel-dungeon
https://store.steampowered.com/app/365900/Pixel_Dungeon/
Unfortunately there is no proper way to play the game on iOS currently.
Happy birthday