Over the last two weeks, we shared Part 1 and Part 2 of this series, which detailed some information about the various releases from Path of Exile's Closed Beta in 2012 through to the Breach release in late 2016. Today's entry covers the remaining past expansions from Legacy through to Delve.

Legacy League (2.6.0)

In early 2017, we were hard at work on The Fall of Oriath (3.0.0), but its beta was still at least a few months away. Legacy League was the perfect challenge league for this slot because it was both relatively easy to develop (remixing a lot of our past leagues) and it celebrated Path of Exile's history up until the end of the 2.x.x series, before The Fall of Oriath introduced significant changes.

The 2.6.0 release also marked the introduction of Solo-Self Found mode.

Legacy League was our most successful release to date, though would be dwarfed by the subsequent one:

The Fall of Oriath (3.0.0)

This expansion remains the largest and most significant update to Path of Exile. It introduced Acts Five through Ten and removed the repetitive "difficulty" system, making Path of Exile one play-through before you reach end-game maps. The community's reaction to the surprisingly large scope of this expansion remains one of the highlights of our time developing Path of Exile.

We worked on 3.0.0 for more than two years before its release, with the final decision to remove the "difficulty" system and create acts Six-Ten occurring around half-way through this development cycle.

This expansion launched in August 2017 alongside the Harbinger League. This was a relatively low-key league that introduced some interesting currency items and a unique map with a difficult but rewarding encounter.

Shortly after this launch, Path of Exile made its console debut on Xbox One, welcoming a wave of new community members!

War for the Atlas (3.1.0)

Not long after our largest expansion ever, we ambitiously set out to release another only four months later. In December 2017 we launched the War for the Atlas expansion which introduced new maps, reworked the Atlas system and added new Atlas mechanics. It also included the Elder, Elder Guardians, Shaped and Elder items and 32 new maps.

Alongside this expansion we launched the Abyss League which followed a similar principle to the Breach league with a thrilling mechanic and deep end-game encounter. Perhaps the most notable element of the Abyss League was the introduction of Abyss Jewels which can be socketed into items for powerful results.

After the release of War for the Atlas, we vowed to get back on our 13 week release schedule and managed to stick to it for all of 2018.

Bestiary League (3.2.0)

In early 2018 we planned to release a set of four leagues that contained new NPCs, in preparation for the unveiling of Path of Exile: Betrayal late in the year that would use these NPCs to replace the old 2014 Forsaken Masters.

In March we launched the Bestiary League, Path of Exile's first "third-generation" league.

The Bestiary League deviated somewhat from traditional Path of Exile leagues and allowed us to explore a new type of mechanic. It was released to a mixed reception, and we reworked many elements of it to be more enjoyable. Thankfully, Einhar Frey captured the heart of the Path of Exile community and was looked back on fondly when the league ended.

The 3.2.0 release also saw a significant rework to the Ascendancy classes.

Incursion League (3.3.0)

Incursion (June 2018) was the second of our NPC-focused leagues with experimental mechanics that we developed in the lead-up to Betrayal. Players enjoyed honing and pillaging their version of the Temple of Atzoatl, and were pleased when we returned this content to the core game months later.

This update was released alongside the introduction of many new Vaal Skills as well as a significant rework to many existing skills.

Delve League (3.4.0)

Delve was 2018's third experimental league and featured an entirely new style of gameplay. Both of the two main characteristics of Delve introduced significant development challenges to overcome:

The Darkness Mechanic
The core mechanic of Delve involves escorting The Crawler through the dangerous darkness of the Azurite Mine. It took a lot of work to get this to feel just right, with the correct mix of tense combat on the path and tempting rewards hiding in the darkness off the path. We were very glad that our development schedule allowed time for iteration with this system.

The significant danger posed by the darkness of the Mine meant that we could justify filling it with valuable rewards. Delve is arguably some of the most-rewarding content in Path of Exile, which is why the rate of Sulphite acquisition in the core game is approximately 37.5% that of when Delve was active. There's ongoing internal debate about whether further rebalancing should tweak the rewards but make it easier to access the content for longer.

The Generation of an Infinite Dungeon
Every player needs a different Azurite Mine, which can span arbitrarily far in three directions. It's also important that the mine is fully-connected. Because we can't generate these in advance (the full mine is too large to store for millions of players), we had to develop a very complex procedural generation system that guaranteed connectivity by nesting different-sized spanning trees together. This was made more difficult by the requirement that we might need to tweak balance values related to the spawn rate or allowable depth of various encounters as the league progresses, without damaging people's existing mines.

That's the end of our look at all the past Path of Exile releases from Closed Beta through to Delve! We look forward to revealing more details of what went into the development of your favourite Path of Exile content in future months.