The wisdom of the past is written on the bones of ancients.

Treeshaker Chimera

All creatures able to block this creature do so.
When this creature dies, draw three cards.

The List

New Capenna Commander

All creatures able to block Treeshaker Chimera do so.
When Treeshaker Chimera dies, draw three cards.

Theros Beyond Death

If a creature can't block Treeshaker Chimera for any reason (such as being tapped), then it doesn't block. If there's a cost associated with having it block, the player isn't forced to pay that cost, so it doesn't have to block in that case either. If there's a cost associated with having it block Treeshaker Chimera, but not to block other attacking creatures, that creature is free to block one of those other creatures or to not block at all.

If a creature is required to block two or more different creatures, its controller chooses which one that creature blocks.