To unlock the Strand subclass in Destiny 2 , you need to own Lightfall and finish the full Lightfall campaign. After the final mission, go to the Tower to speak with Zavala and Ikora, then head to the Hall of Heroes on Neomuna and meditate at the Pouka Pond to permanently unlock Strand.

What to do

  1. Buy the Lightfall expansion.
  2. Play through and complete the Lightfall campaign.
  3. Go to the Tower and talk to Zavala and Ikora.
  4. Travel to Neomuna.
  5. Visit the Hall of Heroes and meditate at the Pouka Pond to unlock Strand.

Upgrading Strand

After you unlock it, you can earn Strand Meditations by using Strand and completing activities on Neomuna. Those meditations are spent at the Pouka Pond to buy Aspects, Fragments, and grenade options.

Extra note

On alternate characters, you usually need to complete the campaign again to unlock Strand there too, unless you use a campaign skip option if available.

TL;DR: finish Lightfall, then meditate at the Pouka Pond on Neomuna to get Strand.