If you put Bound in Silence onto the battlefield without casting it, perhaps using the "recruiting" ability of a card like Amrou Scout, it will enter attached to a creature. You choose that creature as you're putting the Aura onto the battlefield. This doesn't target that creature, so you could have it enter the battlefield attached to a creature an opponent controls with hexproof, for example. If there's no creature on the battlefield it can be attached to, it stays in whatever zone it was in.

If Bound in Silence becomes attached to a creature that's already attacking or blocking, the creature continues to attack or block as normal.

This cards was originally printed with the "tribal" card type. That card type has been replaced with "kindred". This change does not affect the gameplay function of this card.

While it appears only on cards that already have other card types, kindred is a card type and will be counted by effects that refer to the number of card types among cards in a zone.

Kindred is a card type that allows noncreature cards to have creature types. For example, Echoes of Eternity is an Eldrazi (although not a creature) while on the battlefield and an Eldrazi card (although not a creature card) in zones other than the battlefield.