I'm trying to enter in a magic item my DM handed out, but haven't been able to figure out how to get it to behave how I need it to. The basics are:

1. It should provide a +1 to attacks
2. It should provide a +1 to damage
3. It should do 1d4 pierce damage (dagger)
4. It should do 1d4 necrotic damage
5. It should be Finesse, Light, and Thrown

All I can ever get out of it is 1d4 +1 damage when I click to roll the damage. The description will show the 1d4+1 pierce and 1d4 necrotic, but only rolls 1d4.

I added the following Modifiers:

1. Weapon Property - Finesse, Dex
2. Weapon Property - Light, Dex
3. Weapon Property - Thrown, Dex
4. Bonus - Melee Attacks, Fixed value 1
5. Damage - Necrotic, 1d4, Dex, Fixed value 1

You can't have two damage types or rolls on a single weapon. Daggers already have those properties, so you don't need to add them in the homebrew.

Start with a dagger, +1 as the template and just add the following:

Modifier: Damage->Subtype: Necrotic->Dice Count: 1->Die Type: d4

That will give you almost everything you want, but the dice roller isn’t smart enough to roll both damage types together at the same time yet. It will have a note about the Necrotic next to the normal damage type, and you will have to remember to roll that individually. Everything else will be perfect however.