Riding the Storm

Details
Start point	Awaken your latent power of [Arc] Arc Light by generating Orbs of Power and defeating combatants to earn points; using [Arc] damage grants substantially more points. Orbs of Power can be generated by casting your Super. Energy weapons deal elemental damage. Arc element weapons can be identified by selecting them in your Character screen and looking for the [Arc] symbol.
Requirements	A Guardian Rises Quest New Light Activity obtained fromLearning Light Quest A Guardian Rises Quest (see above) New Light Activity obtained fromNew Light Activity obtained fromSchism Activity A Guardian Rises Quest (see above) Beyond Light
Rewards	New Subclass: Stormcaller Storm's Surge New Subclass: Arcstrider Strider's Slash New Subclass: Striker Striker's Slam Arcstrider Stormcaller Striker
Required for	Arcstrider

Riding the Storm is the New Light quest that unlocks the Arc subclasses. It is given by Ikora Rey in the Tower after A Guardian Rises. Sources list it as a Season 12 quest from the Beyond Light era. It takes roughly an hour of general play.

Awaken Arc Light. Earn points by generating Orbs of Power and defeating combatants; Arc damage gives substantially more points. The in-game objectives are Arc points and generating Orbs of Power by casting your Super. The Destiny 2 Wiki describes the step as 1,000 Arc kills, 10 Orbs of Power and 25,000 XP; the data here shows 1,000 Arc points and 3 Orbs, so the requirements may have been revised. Equip an Arc energy weapon to speed things up.
Speak with Ikora Rey in the Tower.

Subclass unlocks: Stormcaller (Warlock), Arcstrider (Hunter) and Striker (Titan) per the game data; sources written at launch list only the first two.
Emblems: Storm's Surge, Strider's Slash and Striker's Slam (Legendary).

Quest steps (from game data)