The Mega Stone required to Mega Evolve Baxcalibur can be obtained from the Z-A Battle Club as a reward for getting promoted in online Ranked Battles.
Nintendo Switch Online membership (sold separately) and Nintendo Account required for online features, including Ranked Battles.
You can participate in the Z-A Battle Club in both Pokémon Legends: Z-A and Pokémon Legends: Z-A – Nintendo Switch 2 Edition.
There are three types of rewards obtainable from Ranked Battles: battle rewards that can be obtained after each battle, promotion rewards obtained upon rising in rank, and season rewards obtained at the end of each season, which will vary depending upon your rank. Keep battling and refining your skills to try to get a variety of rewards. You may receive Mega Stones as rewards for being promoted up the ranks.
Season 4
Schedule: Following Season 3
Rewards: Baxcalibrite
• Baxcalibrite stones cannot normally be obtained during regular gameplay.
• Mega Stones obtainable as promotion rewards are planned to be redistributed in future seasons.
• It is not possible to have more than one of each kind of Mega Stone. If you already have one of any particular kind of Mega Stone, you will be unable to receive another as a Ranked Battles reward.
• For more information, please check the in-game news.