Taiyu-in is the funerary temple of Tokugawa Iemitsu. It’s located in Nikko next to Tosho-gu. It became the standard for all shogun graves.
Not much to say on the topic of Toshogu (and to some degree, Taiyuin) because so much has already been said. The real meat of this series will be in the shrines that we can’t see today.