Custom magic items
 

I got a Potion of Fire Resistance from the Special Items menu, but I can't change the energy type (ref. Potion of Resistance, any energy type, from the Core Rulebook p. 563).

Otherwise is there a way to create a custom magic potion like HLC does?

Custom magic items are more generalized for greater flexibility in HLO (also due to a lot of the changes with PF2, such as not being so tied to spells), so you'd just select "Custom Magic Item". That said, we'll take a look into options for handling Potions of Resistance.

Adding custom magic items is easy, but aside from giving it a name, there doesn't seem to be a way to add any properties to it.

Specifically, we are playing Extinction Curse book 2 (I've purchased the EC content in the HLO store) and I have the Moonstone Diadem that gives me a 1+ to religion checks. It seems like this item should be included in the content, but it's not. So, I've created a custom magic item called "Moonstone Diadem" but aside from naming it there is no way to give it any specific properties that effect my character.

Am I missing something?

Do you have access to the Extinction Curse content? The Moonstone Diadem has already been added.

Thanks, I found it! I had "Hide Invalid Choices" checked, so the Diadem wasn't showing up because it is of uncommon rarity.

To answer my other question, am I missing something or is there no way to add functional properties to custom magic items? I imagine this would be challenging to implement.

It would be very challenging to implement. But you should be able to mimic a lot of a magic item's effects with adjustments.

Just to follow up, I would like to create a custom magic staff as outlined in the GMG. Can you enable the ability to add spells to staves? This feature already exists and can be toggled on with the Staff Nexus wizard thesis, but can you just make it available to all staves to allow custom staves?

That would interesting if they did that for any magic item. Still trying to figure out how to create custom items but I think for me it'd just be adding spells to a mundane item, like a magical hooded lantern that can cast Light. Anything non spell related can go in the description field.
Just starting to figure out how to create custom magic items in HLO as I get more familiar with home brewing them for PF2