Narwhals live only in cold water, so you need to reach a frozen ocean biome before you can tame one.
Finding Narwhals
1. Head toward frozen ocean biomes. These are the icy seas with floating ice and packed ice sheets on the surface.
2. Bring a boat. Breaking through or sailing around the ice is much easier than swimming in open cold water.
3. Look for horned mammals swimming below the surface. Narwhals spawn naturally here, and their tusks come in different sizes, so not every narwhal looks the same.
4. Bring water breathing potions, or plan to surface often. Narwhals swim freely and you may need to follow them under the ice.
Taming Steps
1. Collect a good supply of fish items before you set out. Fishing with a rod or catching fish in buckets works. Carry more than you think you need.
2. Swim or sail close to a wild narwhal.
3. Hold the fish in your hand and use it on the narwhal. Keep feeding until it accepts you as its owner.
4. Once tamed, the narwhal becomes your companion and can be switched between its three behavior modes: wandering, resting, and following.
Tip: Tamed narwhals swim naturally and do not teleport to you across long distances. Tame narwhals close to where you plan to keep or use them. A narwhal tamed far out at sea will not appear at your base on its own.