If the creature you don't control is an illegal target as Signature Slam tries to resolve but the creature you control is a legal target, you'll still put a +1/+1 counter on the target creature you control
This reduces their options while giving you information on their deck, and the creature lets you play the exiled card
As a note, all the information above holds for 40 of the 41 legendary creatures in the set
The cost to play it is also rather small thanks to its low mana value
For the Malaysian market, a box of Collector Boosters from the Commander Masters set (which includes four boosters) costs a cool RM1,099.99, with individual boosters priced at RM249.99
While these cards play best in 3-color decks, they work in 2-color lists, offering cheap acceleration that applies to most cards in a given list