Call to Arms is a weekly challenge given by Lord Shaxx in Destiny 2.
Overview
Players must complete Crucible matches. A single completed match grants 25% towards the goal, with a small bonus for kills and assists; an average match should net around 30%. Completing the Call to Arms rewards one Luminous Engram, which can drop Nightfall-quality loot.
Do i need to speak to Shaxx to activate or will it automatically start counting after the weekly reset?
The PvE equivalent of this activity is the weekly Flashpoint.
Notes
Destiny 2 update 1.0.3 altered the amount of Crucible matches necessary to complete the Call to Arms. Originally, every win granted 10% towards the goal, and every loss granted 7%. Player participation, measured by kills and assists, could boost that percentage slightly. In all, an average match would net around 10%, though it was possible to complete the Call to Arms in fewer than 10 matches.