Complex sentence: Sesshoumaru's mother was entrusted with Meidou-seki (at the instruction of Inu no Taisho) and told to use it when Sesshoumaru came to see her.
Split into simpler sentences: Sesshoumaru's mother holds the Meidou-seki, which was given to her by Inu no Taisho. She was told to use it when Sesshoumaru came to see her.