I hide a Pyke then I played Bone Skewer drag down a thousand tail. Can I activate my Pyke as a hidden card?

No, you cannot activate your hidden Pyke in response to your own spell (Bone Skewer).

Here is the breakdown of why this is not possible:

1. Timing Restrictions

According to RiftJudge FAQ #3046 and #1875, you cannot play or activate a hidden card on the same turn that you hide it. You must wait until at least the next turn to play it. Even if you have the priority to play a [Reaction] at that moment, the "hidden" property of the card does not grant you the permission to play it until the turn after it was hidden.

2. "Activating" vs. "Playing"

It is important to clarify terminology:

• Hiding/Playing from Hidden: "Playing" a hidden card (revealing it) is the action that puts it into play. The rules for playing from hidden state that you ignore its base energy cost (Rule 811.1.b).
• Activating: This refers to using an Activated Ability (the kind with a colon : ). Pyke’s legend ability (which you might be referring to) or other activated abilities cannot be used while the card is still hidden (facedown).

3. Chain Sequence

When you play Bone Skewer:

1. Bone Skewer is placed on the chain.
2. The state becomes Closed.
3. You and your opponent now have the opportunity to add [Reaction] speed effects to the chain (e.g., if your opponent had a [Reaction] spell).
4. Because you hid Pyke this turn, it is not yet eligible to be played or activated.

Summary: You must wait until your next turn to reveal and play your hidden Pyke. You cannot use it on the same turn it was hidden, regardless of what other spells or units are being played.