Pre-release data. All data is from the closed beta and community sources. It is incomplete and will change before and after launch.

Anima Codex
Handbook

Nexus Strand Selection Phase

Combat

When you begin the 10th battle of an Apex Club challenge, the Nexus Strand selection phase kicks in. You're shown three Nexus Strands and must pick one to attach to an Anima already on the field. Drag its icon onto your target to grant a buff. The effect lasts only for the current challenge, but if you win, you keep that Nexus Strand permanently.

Nexus Strand Selection PhaseNexus Strand Selection PhaseNexus Strand Selection Phase

Confidence: unconfirmed · Source: the in-game Handbook (via the Fandom wiki). Pre-release; may change.