As for what this Supporter actually does, it makes all of your Pokemon, Active and Benched, take 30 less damage during your opponents next turn
If you look at Venusaurs ability, it says that you can draw until you have four cards in your hand AFTER your attack
Im sure that this card can draw you plenty of cards in any deck focused on typal themes, which is why it fits so well into the Sliver Swarm precon
Essentially being Panharmonicon on a stick, but turned up to 11, Elesh Norn, Mother of Machines, can completely take over games
Bloodghast serves as a blocker, attacker, and even sacrifice or discard fodder
The random nature of what you can find (or if you hit anything at all) makes this card much more fun in a casual setting, especially for your opponents