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Pokemon Legends: Arceus completely changed the formula of mainline releases in the franchise when it comes to gameplay, while also drawing inspiration from Pokemon Sun and Moon's addition of regional forms. Regional variants were introduced in Generation 7, but they were also reprised in Pokemon Sword and Shield. It makes sense for Pokemon Legends: Arceus to feature Hisuian forms for some Pokemon, and one of the new ones is Hisuian Lilligant - which has a specific evolution method to get.
How to Evolve Petilil into Hisuian Lilligant
The only way to obtain Hisuian Lilligant in Pokemon Legends: Arceus is to evolve a Petilil with a Sun Stone. There are no other requirements, such as Ursaluna's evolution depending on an item and the lunar cycle, meaning that the two main issues might be to find a Petilil and a Sun Stone in the first place.
Where to Find Petilil
Petilil is only available in one of the sub-regions of Hisui, the Crimson Mirelands. Within the Crimson Mirelands, Petilil only spawns in the Holm of Trials area and in the Cottonsedge Prairie, both of which can be somewhat complicated to get to the first time around.
Holm of Trials is where Pokemon Legends: Arceus players can catch Goomy and Hisuian Sliggoo early, too, making it a fantastic area to visit if players intend to have both Hisuian Lilligant and Hisuian Goodra on their team. To reach the Holm of Trials, players would normally have to use Basculegion, but there is a specific location they can use as their starting point and jump across the river with Wyrdeer.
Cottonsedge Prairies is found on the other side of Lake Valor, which players can circumnavigate by following its shore from the east side, near the Diamond Settlement. Cottonsedge Prairies is also home to other interesting Pokemon Legends: Arceus creatures, such as Alpha Vespiquen, Togepi, Togetic, and of course, Petilil.
How to Get a Sun Stone
Much like many other evolution items, Sun Stones can be acquired in a few different ways. The first is to ride Ursaluna and look for buried treasure, which can yield plenty of rare items, including Pokemon Legends: Arceus' new Linking Cord.
The second method is to seek space-time distortion zones, which can sometimes have rare items on the ground - these will have a round shape that's different from the regular Red, Green, and Blue Shards that normally appear.
Sun Stones are also available from Simona's shop in Jubilife Village for 1000 Merit Points, but it's worth noting that the only Pokemon in Pokemon Legends: Arceus that evolves through a Sun Stone is Petilil, making it an item that's better to obtain through other means.
Lastly, players can complete Request 85: At Home under the Eaves, which will provide them with a Sun Stone as their guaranteed reward.
Pokemon Legends: Arceus is available for the Nintendo Switch.