Dedicated counter hero.
A hero whose entire purpose of existence, is to eliminate another specific hero in the game.

The table below explains its feature.

For example:
Amid, is a "General Counter" to Celine.
She nullifies Celine's S2, but does not obsolete Celine entirely.
On the other hand, when there are no enemy Celine, she functions too.

HC42, is a "Dedicated Counter" to Celine.
Its S3: Cast a trap non-attack skill.
Any enemy heroes who have their single target attack skill triggered by this trap will be instantly executed.
Celine is the only hero who will have single target attack skill triggered by a non-attack skill.
HC42 is extremely strong when opponents have Celine, but completely trash when they don't.

(See Hero Concept 42: https://page.onstove.com/epicseven/global/view/9384002 )

Now let's say a hero with its only skill:

"When an enemy dark elemental warrior hero passively heals when getting hit, destroy it".

All this hero does, is  wiping Apocalypse Ravi off all Arena/Guild War defense teams.

It does not change her skills, you can still use A.Ravi in attack teams, or in RTA since you can ban her counter.
This is, like the title says, a way to nerf heroes without changing their kits.