Greetings.

I am creating a series of custom abilities for each player in my upcoming campaign. Example:
Echo of Sorrow

• Effect: Once per long rest, the Bard can use their reaction to empower an ally’s last attack, causing it to repeat with the same effects. This can apply to physical or spell effects (e.g., damage and conditions). The repeated attack mirrors the damage, push, or other effects of the original.
• Limitation: Spells of level 4 and below do full damage when echoed, spells of level 5-8 deal half damage, and level 9 spells deal ¼ damage.
• Flavor: The Bard’s words pulse with energy, echoing the momentum of an ally’s strike and amplifying it.

Is the best way to do this by creating a Feat? Custom spells and attacks I can manage, but I'm not sure how to properly implement this, including the box to check that it has been used. Any input is appreciated.

Thanks!

Greetings Kymagician,

I'll start by saying you might get better responses if you post this in the Homebrew & Houserules Forum.

Now, onto your question :)

You have a few options for doing something like this.
Yes, you could create these as a Feat, then grant that Feat to your Players as you wish.

I am currently playing a character that received a Dark Gift.
These (still) do not exist in DDB, so I created a Ring that has all of the effects and description of the Dark Gift so it would populate correctly on my character sheet.
You could do the same with your 'custom abilities' then you can grant the player the item and they would not see the description in the list of available Feats.

Just a suggestion, and I'm sure there are more :)

Cheers!