This is the list of available monsters from @The Eternal Menagerie that combatants can fight on @The Arena Floor.
Tier: 1 "The Proving Gauntlet" CR 1-4 {
[Black Pudding], [Displacer Beast], [Drow Warrior], [Flail Snail], [Gelatinous Cube], [Gnoll Archer], [Gnoll Hunter], [Gnoll Warrior], [Imp], [Minotaur], [Mummy], [Nilbog], [Ogre], [Owlbear], [Quasit], [Red Knifelin], [Warg]
}
Tier: 2 "The Clash of Realms" CR 5-8 {
[Armanite], [Barlgura], [Blue Slaad], [Canada Goose, Terror of the Realms],[Dolorae], [Drow Mage], [Echo of Demogorgon], [Githyanki Knight], [Gladiator], [Gluttonous Maw], [Gorgon], [Gralnok, the Chainbreaker Ogre], [Green Slaad],[Hag], [Hezrou], [Hill Giant], [Hollowfang Stalker], [King Croakus], [Medusa], [Mind Flayer], [Red Slaad], [Spawn of Kyuss], [Star Spawn], [Sylith, the Venomwing Manticore], [Tanarukk], [The Ashen Hound], [The Bloomed Knight], [Tlacatecolo], [Troll], [Vorpal Rabbit], [Vrock]
}
Tier: 3 "The Legends Bracket" CR 9-14 {
[Ancient Bronze Dragon], [Arch-Hag], [Ashen Colossus], [Balrog], [Behir], [Beholder], [Cloud Giant], [Death Slaad], [Gene-Forged Super Soldier], [Glabrezu], [Gray Slaad], [Harlequin Behemoth], [Lich], [Maw of Yeenoghu], [Sphinx], [Tharvog, the Ironhide Behemoth], [The Bone Harvester], [The Seer of Ashen Doom], [Xytheris, the Storm Serpent]
}
Tier: 4 "The Ascendant Trials" CR 15-20 {
[Adult Obsidian Dragon], [Ancient Emerald Dragon], [Ancient Sapphire Dragon], [Balor], [Chrysalith, the Mind Eater], [Daimao], [Ghorrax, the Arena Butcher], [Hertilod], [Kaelthar, the Shattered Colossus], [Noctyra, the Hope-Devouring Spider], [Seraphiel, the Fallen Flame], [Stonebound Guardian], [Storm Giant], [Thalyth, Mirror of a Thousand Faces], [Tharuun, Herald of the Void Choir], [The Painted Ronin], [The Tyrant Pickle], [Velithra], [Vorgrath, the Abyssal Devourer]
}
Tier: 5 "The Eternal Crown" CR 21-30 {
[Ancient Amethyst Dragon], [Ancient Diamond Dragon], [Ancient Gold Dragon], [Ancient Red Dragon], [Ancient Ruby Dragon], [Ancient Silver Dragon], [Aphoros, Arbiter of Silence], [Astral Dreadnought], [Chalyth, Warden of the Ashen Gate], [Demogorgon], [Devourer of the Unworthy], [Eclipsera], [Elder Brain Dragon], [Empyrean], [Marut], [Myrrakar, the Soul Devourer], [Nytheris, the Frozen Cataclysm], [Oblivoros, Eater of Tomorrows], [Ozyth, the Infinite Maw], [Solaris Rex, the Sun Tyrant], [Tarrasque], [The Arena’s Final Judge], [The Eternity Serpent], [The Herald of Annihilation], [The Scarlet King], [Veyora, the Stormborn Leviathan], [Zeroth, the Living Apocalypse], [Zorvath, the Crown-Eater]
}