Curse of the Allflame: League Mechanic Summary
Overview
Guide
Curse of the Allflame: Gameplay Overview
Path of Exile's 3.29 league mechanic is Curse of the Allflame, a journey to the darkest, deadliest depths of Wraeclast's seas in search of loot.
Find new Encounters in Areas
During normal gameplay you'll find new encounters in areas with treasures guarded by swarms of undead, rival corsair groups, or creatures of the deep.

Treasure guarded by Monsters found in areas
Successfully defeating the Monsters of these encounters will not only provide access to the trove, but also grant a bonus based on the type of Monsters slain. For example, killing undead will possess your character with ghostly power for the remainder of the zone.
Within these troves you'll be able to get your hands on Charts, new items similar to maps that will grant passage to the depths.
Board The Sovereign & meet Valerie

The Sovereign, Valerie's Ship
Valerie is a corsair and captain of The Sovereign, a ship you'll be boarding to venture deep into the seas. Valerie will introduce you to Vesper, a mysterious soul powering the ship. Vesper will be able to get you to where you need to go, and to do that you'll need to make use of the Charts you've come across by using them in a device somewhat similar to how you would access a map.

The Bathysphere will provide passage to the depths
Using this device will essentially open a portal to a new area, the ocean floor, where you can explore in search of loot. Beware however, these ocean encounters have new mechanics and dangerous new enemies.
The power of the Allflame
So that you don't immediately drown, you'll need to use the power of the Allflame by placing Allflame lanterns - areas that you can move freely within while on the ocean floor. The areas are clearly marked with green rings signalling their edges. The first is created around the bathysphere on first entering the area, and the rest can be placed, either manually or automatically as you move outward of another lantern area.

The lantern area surrounding the Bathysphere
Be aware that your only way out is back into the bathysphere, so once you've dropped all of the Allflame lanterns, you'll be on a timer before the lanterns run out of energy. Finish up your tasks, pick up any valuable loot and head back to the bathysphere to avoid drowning!
Find Valuable Loot and Resources
In ocean areas, be on the lookout for strange, ghostly looking coral formations. The true purpose of your excursions to Wraeclast's ocean depths is to extract these mysterious findings because they can be converted into a new resource: Dead Man's Sulphur, the very fuel that powers The Sovereign.

Strange, glowing growths on the ocean floor
Fully exploring ocean areas is important to ensure you can harvest as much Dead Man's Sulphur as possible to continue your journey, as the next step involves planning a larger Voyage.
Plan a Voyage

Valerie keeping track of your Charts
As you progress through Chart areas, you'll be able to use those explored regions to plan a Voyage, which is essentially a much larger excursion to the ocean floor in which you'll be revisiting all of the previously completed Chart areas again, all at once, with even more reward bonuses on top.

Build a Voyage from your Chart areas
To do this you'll need to plan your Voyage using the Voyage board, by placing charts in up to 9 slots. Each Chart area will apply special bonus modifiers, either to adjacent area or the entire Voyage, to boost your loot found within that Voyage. Additionally, the edges of the Voyage board will contain even more bonus modifiers to boost loot found in area that connect on the board.
Because Voyages contain up to 9 Charts, there's a lot of potential loot to pick up. To deal with this, you'll be able to fill special Allflame Capsules during the Voyage which will be sent up to The Sovereign where you can reclaim it upon resurfacing.
A New Form of Crafting: Allflame Crafting
The Dead Man's Sulphur that you've been stockpiling to power The Sovereign isn't just for that purpose, as it will also enable you to make use of a brand new, powerful crafting mechanic: Allflame crafting.

Place your item and crafting currency into the Allflame Crafting window
This new crafting system is somewhat similar to a Hinekora's Lock. You'll be able to place your item into the window along with the currency you wish to use on it. Instead of providing a single outcome like it would have done if you simply performed the process normally, it will provide you ghostly visions of several different outcomes.

Review the outcomes and make your choice
Once you've spent your Dead Man's Sulphur on the craft, you'll be able to freely choose between the outcomes to receive your upgraded item. This process can be performed for almost any crafting step, including things like Influenced Exalted Orbs, pre-loaded Resonators and even Fracturing Orbs.
Beyond this new crafting system, the Allflame Crafting mechanic can also be used with new Ducats that provide additional tools, such as Kishara's Ducat which can split an item into many ghostly forms, separating its modifiers onto each.

The new Kishara's Ducat