The War Within: Journey's End (Season 2)

The most efficient way to get Delver's Journey rep is by doing one weekly (bountiful?) delve for the weekly quest The Key to Success it is a guaranteed drop from the chest at the end of any delve on any difficulty. Reaching the max level of the Delver's
Journey in The War Within Season 2 also rewards the Nightfall Sanctum Warband Campsite. After the weekly quest you can continue to progress the Delver's Journey by spamming delves, but at a tremendously slow pace.

Despite The War Within Season 1 having a Delver's Journey, there was no achievement for reaching the max level of the season 1 Delver's Journey. So this the first achievement of its kind.

This has just popped for me and no idea what I did to get it

Every delve completed gives progress towards a seasonal pass.