Hekili is not compatible with the Midnight (12.0) expansion.

What is Hekili?

Hekili is a priority helper addon that can make recommendations several steps into the future. It works by collecting information about the game's current state, using a SimulationCraft-like action priority list (APL) to recommend the next ability to use, then modifying the game state and making the next prediction.

All DPS specializations are currently supported as well as all tanking specializations. 

Action Packs

Action Packs are shareable instructions that direct the addon how to make recommendations.  They use action priority lists similar to SimulationCraft, and in most cases, are imported directly from SimulationCraft.  Action Packs only work for specs that are currently supported by the addon.  You can view Action Packs via /hekili > Action Packs.

Alternate Customization

Example:

Above is the default Primary display, with 4 icons visible.  The Current Mode is set to Auto, so this display will automatically adjust to the number of enemies that are detected by the addon.

This shows both displays, because Current Mode is set to Single Target, forcing the Primary display to pretend there is only 1 target no matter how many are detected. Under those circumstances, the AOE display appears to give you these alternate recommendations.

Note on 'Current Mode' Setting

In the vast majority of cases, the Current Mode: Auto setting will be optimal. However, sometimes there are special mechanics in a fight that involve wanting to focus on one enemy at a time rather than maximizing DPS across multiple targets. In this case, it can be helpful to force both displays to be shown. You can set a keybinding to toggle this setting while in-combat. Bind a key for Mode Switch under /hekili > Toggles.

Other Customization

You are able to customize displays further. You can change the direction of your recommendation queues, modify its alignment, turn on Blizzard's overlay glow for the first recommendation icon in your display. You can specify that up to 10 icons are shown (more recommendations equals more processing power, though!), change the spacing, the zoom, the primary and queued icon sizes. You can specify the font and font sizes. You can specify that some additional information should appear on the base of the primary icon. You can use SpellFlash if you have that addon loaded as well.

You can use the Masque icon to apply a custom style to your displays.

You can specify visibility in PvE and PvP settings with custom alpha/transparency settings for each. You can specify special Conditions using the SimulationCraft-like syntax used in the Action Lists (this is how the Specialization AOE displays are scripted to see if your Current Mode is "Single Target").

A display like the above can be created by going to /hekili > Displays > Specialization Primary > UI and Style and adjusting the following:

1. Set Icons Shown to 2.

2. Set Queued Icon Size to 30.

A display like the above can be created by going to /hekili > Displays > Specialization Primary > UI and Style and adjusting the following:

1. Set Icons Shown to 5.

2. Set Queue Direction to Up.

3. Set Queued Icon Size to 30.

A display like the above can be created by going to /hekili > Displays > Specialization Primary > UI and Style and adjusting the following:

1. Set Icons Shown to 1.

(Hey, that looks a lot like a single-icon priority helper WeakAura!)

Troubleshooting

1. There's nothing showing!

A. Did you use this addon before Legion? You may have old, incompatible data in your profile. Go to /hekili > Profiles and click Reset Profile to start from scratch. You may want to reload your UI at that point as well.

2. There's still nothing showing!

A. /hekili > General: Make sure Enabled is checked. If it's not checked, the addon may have detected an error and shut itself off. If that is the case, use an addon like BugSack/BugGrabber to collect any error messages and submit it via the ticket tracker (linked above). If the addon is enabled and nothing is showing, check your displays. /hekili > Displays > Primary and AOE.  Check the alpha (transparency) settings on the Visibility tab.  If your issue is with one of the default displays, you can create a new profile and see if your displays are shown.