Do I run it in my deck or do I use it as a separate purchase source?
How do I use "soul of..." Proxy cards?
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Do I run it in my deck or do I use it as a separate purchase source?
As described in the Comprehensive Rules, if a legal card has the Keyword "Soul of XXX", where XXX is an older card which is not legal in the format you are playing, then that older card may be used as a proxy for the legal card: it goes in your deck and is played exactly as the legal card.
(If both cards are legal, then you play either card according to its most recent printing, but count all copies of each card towards the one deck construction limit. For example, you can play with 3 copies, in total, of Matsu Tayuko nonexp and Matsu Aoiko nonexp.)