He could not resurrect himself. He needed someone in not-ghost form, to collect some special items and perform a magical ritual. Some of the special items were

He could not resurrect himself. He needed someone in not-ghost form, to collect some special items and perform a magical ritual. Some of the special items were also one-time use iirc. Perhaps, this tool needs to additionally encrypt some of the pieces with the dna of one's father or whatever.