Avatar Aang will be great in that very narrow archetype and useless outside of it
Aeve brings an oozy army to the table, and with plenty of ramp and card draw, youll never run out of slimy reinforcements
Then two-color lands using the multicolor method, the same for tri-lands, followed by any color lands such as Command Tower, Exotic Orchard, and Opal Palace after which colorless lands are at the back of the box, alphabetically
Or creatures with ETBs that get Oblivion Ringed by an opponent
It also helps wipe the board of your opponent's monsters, can't be countered, and stops other cards and effects from being activated
Utility, Sideboard, and Flex Options The above group of cards expands beyond the five shown, but these are the most important options available in terms of tuning Mono-Red Prison to an expected metagame (this is partially a farce due to how wide-open Legacy typically is, but you can still take a general guess at what the winners metagame will be)