To accomplish the Boon Companion achievement you must use certain items with Yorick throughout the adventure of Shadowgate. Items include:

Location: Antechamber
Description: Burn the rug, the coin lies underneath.

Location: Grotto
Description: PIckup from the ground.

Location: Wizard's Den
Description: PIckup from the ground.

Location: Battlement
Description: Sitting on the base of the statue.

Location: West Tower (1st Floor)
Description: Use Yorick on the Fountain.

Location: Library
Description: Use 12th book sitting on the bottom row of the bookcase.