Could you please make Harsh Lessons work with Supports?

The new DoT passive clusters specify Spell Skills, which probably means that Support Gems like Frigid Bond and Decay do not benefit from it.

That's in addition to not getting a damage buff like other skills.

I'm looking forward to trying Frigid Bond with an appropriate Mercenary, but I feel like it's gonna be so far behind the rest of the Cold DoT options.

I don't believe any proper builds are using Decay, so it would probably not matter if it ended up working for it too.

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YiamiYo#3159 wrote:

The new DoT passive clusters specify Spell Skills, which probably means that Support Gems like Frigid Bond and Decay do not benefit from it.

They do. These supports don't deal damage themself. They add a damaging stat to the supported skill. That's why if you have Fireball supported by Decay, Swift Affliction on top would not only scale ignites from fireball, but also decay's DoT.