The Fang of Watatsumi is a subquest of the Moon-Bathed Deep quest. You can find it to the northwest of Sangonomiya Shrine. just open your quest list, then travel to the waypoint when you select the Eye of Watasumi quest under the Moon-Bathed Deep.
Just like the other shrines, this one will be sealed. You will need to open it to interact with it. To do so, read the tablet to the left of the Shrine, then interact with the Ley Line Monolith behind you. This challenge is traditional something people try to do in cooperative, as it is difficult.
You need to defend it from waves of enemies that spawn in for three minutes. Each enemy you kill will knock a few seconds off the countdown. This is tough, as enemies spawn at a solid rate, and will head straight for the Ley Line Monolith seeking to damage it.
If you take out the first waves, then wait for a Hilichurl caster to spawn as part of one of the waves. Take out all enemies in the wave except for the Anemo Hilichurl caster on the left. Occasionally interrupt him and freeze him to try and keep his focus on you.
As long as just this Hilichurl is alive, no other enemies will spawn in, and you can just the timer go down.