- Counter target noncreature spell.
- Return target permanent to its owner's hand.
- Target creature gets -3/-3 until end of turn.
- Destroy target planeswalker.
Giving a target creature -3/-3 is good, although it didn't deal with the more annoying creatures during Standard at the time like Courser of Kruphix and Siege Rhino. But, it hit a lot of things, and as only half of a whole, it's very strong.
The last option to destroy target planeswalker is plenty good. It was a good way to save your copy of Hero's Downfall for something else. As the years have gone on, too, planeswalkers have become much more plentiful. Especially with the glut of walkers War of the Spark brought into the game, this is a mode that's only going to become more important as the years go by.
During its entire time in Standard, Control decks did maindeck one copy of Silumgar's Command, from Esper Control to Grixis Control to Sultai Control. While 5 mana feels like a lot for an instant, this card often does the work of two cards at once. Two-for-ones are an awesome thing. However, five mana for this card isn't efficient enough for Modern. However, with the emergence of the Pioneer format, it's possible that we will see this card see competitive play once again.
However, Silumgar's Command does still see a modest amount of EDH play. The decks that utilize this instant the most include Inalla, Archmage Ritualist, Tasigur, the Golden Fang, Kess, Dissident Mage, and quite appropriately, Dragonlord Silumgar.
As far as price, Silumgar's Command was reprinted in Commander 2017 and is considered a "bulk" rare. In 2019. you can get copies of this Command for $0.25 or less, a far cry from the $1 this card once Commanded during its heyday in Standard. Still, this is a solid card for a control-centered Commander deck or Kitchen Table control decks.