A sideboard is an additional set of cards (usually 15) to accompany your main deck
But if you want a strong Commander deck, you need a good commander, which begs the question: Which theft commander is the best
Cosmic Spider-Man With every keyword under the sun, Cosmic Spider-Man not only dominates combat but also buffs all your spiders with the same suite
Top Cards Madame Null, Power Broker lets Jenova act more like a single-target Ouroboroid, while +1/+1 counters synergy cards like Hollowmurk Siege and The Ozolith let you pile innumerable buffs onto your Creature, Mutant or not
However, there are also five more archetypes hidden in the set, which are harder to draft or build
Remember though, the chase for that card was only in collectors boosters which are much more expensive than regular draft boosters