i want to create a subclass that has the undead warlock subclass ability form of dread. it has loads of effects and im confused, please help

You would just attach a single General action to your subclass with the following parameters:

• Activation Type- Bonus Action
• Reset Type- Long Rest

Then you would include all of the details in the Snippet field for both the Action and the feature itself, as well as the Description field of the feature (you can skip the description of the Action, it’ll default to the feature’s description). You can start off by simply copying and pasting the text directly from Form of Dread. Then change the first bullet point in each of the snippet fields to read as follows: “You gain <strong>1d10+</strong>{{classlevel}} temporary hit points.”

That’s pretty much it. Everything the feature does is only temporary, and temporary effects are not currently supported by DDB, so it just needs to be plain text for the most part. If you wanted to, you could change the second bullet point to read as follows, but it isn’t really necessary: “Once during each of your turns, when you hit a creature with an attack roll, you can force it to make a <strong>DC</strong> {{savedc:cha}} WIS saving throw, and if the saving throw fails, the target is frightened of you until the end of your next turn.”

Thank you, but because this is temporary should I do it as an item?

It won’t really help you. The player will still have to add the temp HP manually anyway, the second bullet point won’t matter because character sheets don’t actually interact with monster statblocks, only the third bullet point would matter.