Shiny Reshiram is Reshiram's shiny form: a different colour, and stronger, with 30% more health and move damage than a normal Reshiram. Shinies are the strongest Pokémon in the game.
How rare it is
As a legendary's shiny, it's one of the rarest Pokémon in the game: 1 in 600,000 for each Reshiram beaten in a raid, and 1 in 1,000,000 on its legend map.
Where to find it
It doesn't defend any map: a Shining Ball (1 in 5 shiny), a Premier Ball (always shiny) or a Master Ball can catch it. See Shinies.
Past level 100
A shiny never becomes a Nightmare form: at 5★ and level 100 it Ascends instead, keeping its shine and its name, and climbs the Nightmare ranks to level 999. See The Nightmare World.