Mere Words to Slay an Olden Emperor!
The utilization of Haki was extremely unique as those weaker than Noah who were oppressed by his will would automatically lose 50% of their battle power against him.
It had turned passive when the Tyrannical Emperor became TABOO as Noah could actively use it against his enemies, and the unique thing was how its resource cost varied depending on the target!
If it was a weak enemy that Noah's will could easily overcome, the resource cost was negligible and not even worth mentioning.
But if it was someone truly powerful that even his strong will and soul couldn't easily sway- the resource cost would be higher the longer he directed this ability against them as it could theoretically reach hundreds of thousands of Units of the Essence of Reality per microsecond!