1. A nature spell?
2. An ability of certain creatures (e.g., a Chaos Dragon or a Berserk Bull)
3. A skill in "combat"?
Depending on this, we need to "adjust" accordingly. Complete antimagic simplifies the game as much as it complicates it. Antimagic as a spell is the best option.
A skill for defending against magic is a plus that any hero can use, provided they have maxed out their melee combat skill. Although any hero who has maxed out their skill by 100% is already quite powerful.