What's the homebrew guidance for a weapon that adds a d10 to your initiative? Nothing I am doing seems to work.

Basis: "A Storm is Coming. While the bow is on your person, you add a d10 to your initiative at the start of every combat."

Try  going to modifier. bonus > initiative,  dice count > 1, dice amount d10. If not well idk.

Yeah, tried that first. Didn't work. Then tried adding fixed value, didn't work. Tried adding dex base. Didn't work. Then tried affering it a free timeshare stay in the Bahamas...

You cannot add a die to initiative, only a fixed value or a value based on either an Ability modifier or PB.

Sad panda. Thanks!

You can make it as an Action that the player would just have to roll separately. It won’t automatically add it to the Initiative roll in the system, but you could do it.

IamSposta,

Is this just something you have to know, or is there some rhyme or reason to this that I could learn?

I have a set of items I create for my campaigns like "Effects of Bladesong", "Effects of Bless", etc. This lets players have those items in their inventory, and then "wield" the effect if appropriate. I've been unable to create "Effects of Gift of Alacrity" to add a 1d8 bonus to initiative, and I would sure like to.

The system is incapable of combining another die roll with a standard initiative roll. Just like the system is incapable of adding a 1d4 from something like guidance to an ability check. You can add a fixed value to those rolls, or even a value based on something like an ability modifier or proficiency bonus, just not a random value like a die roll.