Complex sentence: After his careers and death, his will included donating materials and money for the Yakama Nation to build a facility which was the seed of the Yakama Cultural Center which included his wishes for a library and museum with materials and artifacts he had gathered in his travels.
Split into simpler sentences: After his careers and death, his will included donating materials and money for the Yakama Nation to build a facility. This was the seed of the Yakama Cultural Center which included his wishes for a library and museum with materials and artifacts he had gathered in his travels.