Path of Exile's free Betrayal expansion, which lets you play as a detective (of sorts), has clearly struck a chord with players: soon after it launched yesterday, the ARPG smashed its concurrent player record, and at its peak, 117,000 people were logged in at the same time on Steam alone.
It was the first time the game passed 100,000 concurrent players on Steam, according to Steam Charts, and was more than double its highest player count in the three months prior. Judging by what lead developer Chris Wilson has said previously about the number of players that go through the game's standalone client instead, the total player count was probably closer to 200,000 (thanks to Reddit user Vindexus for pointing that out).
We just broke our all-time concurrency record for players online (and still climbing!)December 7, 2018
Betrayal is a huge update that reworks old leagues into the main game, adds new masters, missions, maps and skills, and unifies hideouts. The detective work comes in when you face the Immortal Syndicate, a group of necromancers. To stop them, you have to hunt down members from the organisation's four divisions and interrogate them—or bargain with them—to learn about its structure.
You can convince them to betray their fellow criminals (hence the expansion name), and you'll end up raiding safehouses full of loot to take out high-ranking members. Eventually, you'll face the mastermind behind the whole thing. As Steven pointed out, it's not unlike Shadow of Mordor/War's Nemesis system.
Path of Exile is free to play on Steam.