If I have a Stellacorn Herder in base, and play a Tideturner in base, can I choose Stellacorn as a target for Tideturner's effect? What happens?

Now I understand the situation. Let me address this:

No, you cannot choose Stellacorn Herder as a target for Tideturner's effect, and you cannot play Tideturner in base in the first place.

Here's why:

1. Tideturner cannot be played in base: According to Rule 737.1.b and the official FAQ, the Hidden keyword allows you to hide a card facedown specifically at a battlefield you control—not in the base. When you later play Tideturner from hidden, it must be played at the battlefield where it was hidden.

2. Even if you could play Tideturner in base: Tideturner's ability reads "choose a unit you control at another location." The base is not a location in the same sense as battlefields. The ability requires targeting a unit at a different location than where Tideturner is played. If Tideturner were in base, it could only target units at battlefields (and vice versa).

3. Stellacorn Herder is in base: Since Stellacorn Herder is also in base, it would be at the same location as Tideturner, making it an illegal target (Tideturner requires "another location").

Bottom line: You'd need to play Tideturner at a battlefield to use its effect, and then you could swap it with a friendly unit at a different location (either base or another battlefield).