The Magic Mirror
Legendary Artifact
This spell costs 
less to cast for each instant and sorcery card in your graveyard.
You have no maximum hand size.
At the beginning of your upkeep, put a knowledge counter on The Magic Mirror, then draw a card for each knowledge counter on The Magic Mirror.
Illustrated by
Throne of Eldraine#345MythicEnglishLegality
Rulings
The cost reduction ability reduces only the generic mana in the relic's cost. The colored mana must still be paid.
Once you announce that you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to change by how much a relic's cost is reduced.
If a split card is both an instant card and a sorcery card, it's only counted once for The Magic Mirror's cost reduction ability.
Adventurer cards aren't instant or sorcery cards while they're in your graveyard.
If The Magic Mirror leaves the battlefield while its last ability is on the stack, the ability will use the number of knowledge counters The Magic Mirror had before leaving the battlefield to determine how many cards you'll draw.
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.