MTG Treachery

The X in the unveiling triggered ability has the same value as the X paid in the Unveil ability. This is pretty easy to derive since there is no other source of X.
Once The Depths Caller’s unveiling triggered ability begins to resolve, no players may take other actions until it’s done. Notably, players can’t try to destroy your permanents after you’ve chosen a type.
If a card in a player’s library has {X} in its mana cost, X is considered to be 0.
You can tap any untapped creature you control, even one you haven’t controlled continuously since the beginning of your most recent turn.

Tapping an untapped creature that’s attacking or blocking won’t cause that creature to stop attacking or blocking.
Tapping an artifact won’t cause its abilities to stop applying unless those abilities say so.
Equipment attached to a creature doesn’t become tapped when that creature becomes tapped, and tapping that Equipment doesn’t cause the creature to become tapped.