How do you get Prismana Fenmane?

The level that counts is your homeland's, not your season or Battle Pass level. Two different "level 10" rewards hand out Prismana eggs, and they are easy to mix up. The Companion Handbook's level 10 egg is a battle pass reward and changes from season to season. The Fenmane egg comes only from levelling your homeland.

So if you want Fenmane, put your effort into the homeland. Season ranks, event goals and side quests will not get you this one.

How the other egg-only Prismana compare

Each egg-exclusive Prismana has its own single route, so working towards one does nothing for the others. Prismana Infergon is a Season 1 reward, earned through stamps in the Season Journey when you reach season rank 12. Prismana Sparkelf comes from an egg tied to an event's personal goals. Prismana Iris is a hidden egg at the end of the Fragrant Journey, a side questline after the campaign that finishes with turning in fragrances once Tide Blossom Coast is unlocked.

Of these, Fenmane is the only one that comes from homeland progress. It does not depend on a season or an event window.