Got a response from Nathan, confirming the diagram in the rulebook, and that Tetherys and Lindel are unfortunately worded:
Hi Paul,
You are correct—the blue die is the attack die and no other die is the attack die.In the case of Tetherys (and Lindel, actually), “attack dice” does mean “attack pool” (all attack dice and power dice rolled). There are a couple times that the rulebook also uses “attack dice” when it should say “attack pool.”Thanks,Nathan HajekGame DeveloperFantasy Flight Games