Thanks to JBS 80 for the verbiage within his comment on the other "solution".
How to legitimately unlock this achievement: it's awarded when the sum of the renown for all of the underworld factions reaches 1000. Renown is awarded for completing increasing level delves in factions, as well as upgrading the level of each faction's treasure horde. Delves are limited to 3 a day so the amount you can earn per day is limited. At the moment there's only two factions, so you'd need an average of 500 renown on each faction to reach 1000, but more factions will be added later (one every 4-5 weeks, I believe) making this achievement easier (i.e. 5 factions at 200 each).