According to the 1.12 patch notes, powerful enchantments have an approximately 20% chance of occurring, and they are about 50% better than normal.
Wandering enchanters are capable of performing powerful enchantments. Wandering enchanters have a random chance of appearing in certain spots in the overworld or in dungeons. The item's original stats will not change. If you wish to free up an item's enchantment slots, you may pay an enchanter to remove all its existing enchantments. There is no chance of accidentally wiping enchantments. See the Enchantment Price Calculator for more details. Each enchantment costs gold, and the cost escalates based on item quality, enchanting NPC, and the number of enchantments on the item. Some enchanters specialize in certain types of enchantments. A basic enchanter appears in each town, but more powerful enchanters can be found elsewhere. There are multiple enchanter NPCs in the world. For example, an enchanter with a maximum of 2 enchantments can only add an enchantment to an item that currently has fewer than 2 enchantments on it.Each enchanter has a maximum number of enchantments they can add to any item.
Each item can only hold a limited number of enchantments, in addition to its original stats.