The Circle of Loyalty

Mana cost: {4}{W}{W}
Type: Legendary Artifact
Set: Throne of Eldraine Promos #9p (mythic)
Price: $1.55 USD

Affinity for Knights (This spell costs {1} less to cast for each Knight you control.)
Creatures you control get +1/+1.
Whenever you cast a legendary spell, create a 2/2 white Knight creature token with vigilance.
{3}{W}, {T}: Create a 2/2 white Knight creature token with vigilance.

The Circle of Loyalty has 6 printings, first in Throne of Eldraine (2019). EDHREC ranks it #7,011 among Commander cards. It appears most in Sidar Jabari of Zhalfir decks (7,817 on EDHREC). 1 public deck on TCG runs it. It has 5 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: legal
pioneer: legal
standard: not legal
brawl: legal

Public decks that run The Circle of Loyalty

Legends' Legacy - Dominaria United, led by Dihada, Binder of Wills

Commanders that run The Circle of Loyalty most

Deck counts from EDHREC.

Sidar Jabari of Zhalfir: 7,817 decks
Syr Gwyn, Hero of Ashvale: 2,827 decks
Dion, Bahamut's Dominant // Bahamut, Warden of Light: 1,974 decks
The Council of Four: 738 decks
Dihada, Binder of Wills: 647 decks
Aryel, Knight of Windgrace: 586 decks
Shanid, Sleepers' Scourge: 534 decks
Haytham Kenway: 363 decks

Often played with The Circle of Loyalty

Cards that show up in the same decks on EDHREC.

Rulings

2019-10-04: The Circle of Loyalty's triggered ability won't trigger when you cast it because it's not on the battlefield yet.
2019-10-04: The cost reduction ability reduces only the generic mana in the relic's cost. The colored mana must still be paid.
2019-10-04: The Circle of Loyalty's triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
2019-10-04: 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.
2019-10-04: 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.