1) Whisper of the Dead
(Comparable to Diablo 3 bounties)
Screenshot:
Completing various objectives (could be dungeons, open world events, world bosses, pvp bosses, pvp altar conversion, etc) will fill up your Grim Favors gauge (displayed at the bottom of the map screen).
The more rewarding objectives are displayed in a deeper red, the lesser in a pale pink. There is a daily/gated component to it, meaning you will run out of deep red objectives if you do them all day.
Once your Grim Favors gauge is full, you have to speak to the Tree in a small town (forgot the name) of Hawezar swamp region (south east of the map, there is a "skull with red branches" icon).
The Tree will reward you with experience and give you a bounty cache (you can choose amongst a few), the cache will have a high chance to drop legendary items and nightmare sigils, among other things.
Difficulty
Easy and chill. Can pick the objectives you want to complete since they are so varied.
Rewards
Nightmare Sigils (actually a good source of them).
Experience.
Legendary items.
2) Helltide events
(Comparable to Diablo 2 terror zones)
Screenshots:
A big area randomly and periodically becomes corrupted (appears red on the map). New and more dangerous monsters, bosses and events will spawn there, including a very strong roaming boss named "Kixxarth The Forerunner" that usually requires a group to be defeated (or a very strong build, to be soloed).
Killing stuff there awards Aberrant Cinders, which will allow you to open chests (Tortured Gifts) scattered in the region, those chests give a specific legendary (the type of which is indicated on the chest) and a unique crafting material called "Fiend Rose", which is not found anywhere else, allowing you upgrade your gear to the highest rank.
Difficulty
Excluding the roaming boss "Kixxarth The Forerunner", the event is largely soloable and overall pretty easy if you have a half decent build, but it's more efficient to do in group. It may be moderately challenging early on at lower level or if your build isn't well optimized.
Rewards
Fiend Rose (crafting material which allows you to upgrade your gear to the latest rank, can't be obtained anywhere else).
Targetted Legendary items (can target farm the type of item you need, since the chests tell you what type of item is offered).
3) World Bosses
(Comparable to Loot Piñatas)
Screenshots:
https://i.imgur.com/jx1gLi5.jpeg
In endgame CBT, World Bosses spawned every hour or so. When they are 30 minutes away from spawning, an icon will show up on the map with a countdown. (I don't know if they will change the frequency for release).
World Bosses always spawn on a timer, in one of their dedicated arena, an empty area on the map which stands out (there are a few scattered across the map), they never spawn randomly/suddenly just anywhere in the open world (unlike what some early official clips show).
If I remember correctly, the icon/timer will only show up on your map if you have explored nearby. It's a good idea to explore various corners of the map early on so you have a portal ready to quick travel to them.
Difficulty
Slightly challenging early on, very easy at high level with a half decent build.
Rewards
Scattered Prism (used to socket your gear, can't be obtained anywhere else).
Legendary items.
4) PVP area farming
Screenshot:
https://i.imgur.com/mizBWSS.jpeg
Now wether you like PVP or not, the PVP area offers one of the most viable way to farm legendary equip (at least this was the case in endgame cbt).
There are a lot of monsters there, which you can farm for Seed of Hatred (or even regular gear, since density is pretty good). Seed of Hatred is a currency which, once converted via the PVP altars, allows you to gamble for gear (also offers a few cosmetics). In endgame CBT it was the most viable way to farm gear, and so most players didn't Bloodmark (pvp flag) themselves so they could cooperate and simply PVE farm.
Bloodmarking oneself allows you to attack anyone (including non-Bloodmarked players), it's basically "hostile mode", and doing so increases your Seed of Hatred rate, but you can choose not to Bloodmark yourself and cooperate with other non-Bloodmarked players to simply farm PVE monsters. This will not prevent you from being attacked by hostile "Bloodmarked" players though.
Difficulty
Extremely variable depending on the hostile Bloodmarked players present. PVE monsters themselves are moderately challenging (easy with a half decent build). The roaming boss Khazra Abomination can be challenging and depending on your class/build, may not be soloed. Playing in group is more efficient and will give you a significant advantage.
Rewards
Seeds of Hatred (allows you to gamble gear and buy a few cosmetics, can't be gotten anywhere else).
Monsters also drop regular gear, including legendary. Density is pretty good.
5) Nightmare Dungeons
(Comparable to Path of Exile maps)
Screenshots:
https://i.imgur.com/ytLwLuU.jpeg
Now I saved the most challenging content for last, and what some will call the "true endgame". Nightmare Sigil is a consumable (which can be found or crafted by the occultist) which allows you to corrupt a dungeon with certain affixes.
Once the dungeon is corrupted, you have to make your way to it by foot or horse on the open-world (but you can be summoned by players already there). You have a limited amount of revives inside the dungeon.
Edit: It appears you're now able to teleport directly to a nightmare dungeon:
Small update to note with nightmare dungeons you should note. In the alpha test late last year when you got a sigil you could use it to teleport to the dungeon so you don’t have to ride or walk there. Otherwise this is a good guide. -/u/Thatdude446
The level of the monsters will depend on the tier of the Nightmare Sigil, for example a Tier 66 Nightmare Sigil will give you level 120 monsters.
The implicit dungeon objectives don't change (yep, you still gotta find the key, activate the switch or bring the stone to open the door, etc), monster density is increased, and some affixes may affect the way you play, for example there is an affix which makes a lightning bolt periodically hit you (it can one shot you), while there is an artefact following you around and periodically casting a dome, you have to go under the dome to not get hit by the lightning bolt. There are probably a handful affixes such as the Lightning dome, which stand out.
Difficulty
Most challenging pve content by far. Difficulty goes up to Tier 100 (while open world content is limited to Torment, aka Tier 4) and reaching this high will require a very good, if not meta build. The Nightmare dungeons are soloable, but playing in group is more efficient, especially since it effectively gives you more revive (you can be revived by players, it doesn't decrease your revive count, so always wait before self-reviving).
Rewards
Glyphs (I don't remember if it's the only source of glyphs).
Leveling up Glyphs (the only way to do so, at the end of a nightmare dungeon there is an altar to level up your glyphs).
Glyph is an item which gives a huge passive bonus, similar to Diablo 3 legendary gems, which is socketed into the Paragon board (each board has 1 slot, and we're looking at 3~5 Glyphs per build).
