His ability is based on his League of Legends ability Judgment. However, the magic immunity effect is unique to Teamfight Tactics.
Due to his ability having multiple mechanics such as magic damage immunity and high amounts of AoE magic damage, Garen can both be built around Ability power items such as Morellonomicon or Jeweled Gauntlet or Armor items such as Thornmail or Frozen Heart.
Since Garen'sJudgment hits multiple times. This makes Jeweled Gauntlet exceptionally effective due to the multiple chances to activate, resulting in a consistent damage boost.
Active - Judgment: Spins his sword around his body for 4 seconds, becoming immune to magic damage and dealing 60 / 90 / 120 magic damage to nearby enemies each half-second. The spins can deal a total of 540 / 810 / 1080 magic damage.