Intensity/Threat:
* Ferocious: Wildly fierce and savage.
* Brutal: Extremely cruel and violent.
* Savage: Fierce and untamed.
* Vicious: Extremely cruel and malicious.
* Ruthless: Having no mercy or compassion.
* Aggressive: Ready to attack or confront.
* Menacing: Threatening or alarming.
* Intimidating: Causing fear or apprehension.
* Daunting: Seeming very difficult or challenging.
* Formidable: Inspiring fear or respect through being impressively large, powerful, or intense.
* Terrifying: Causing great fear.
* Horrifying: Causing extreme fear and disgust.
Intensity/Energy:
* Powerful: Having great strength or force.
* Dynamic: Full of energy and vitality.
* Intense: Very strong or powerful.
* Forceful: Showing strength and determination.
* Vigorous: Full of energy and enthusiasm.
* Energetic: Full of energy and activity.
Other related words:
* Dreadful: Inspiring fear or dread.
* Fearsome: Causing fear.
* Grim: Unpleasant or depressing.
* Stark: Severe or harsh.
* Unrelenting: Not giving up or stopping.
* Uncompromising: Not willing to change or compromise.
The best word to use will depend on the specific situation and what you want to convey.