Appearing first in Shards of Alara, and also included in a Planechase pre-constructed deck, Quietus Spike is a 3-drop Equipment that gives a creature deathtouch. Also, whenever that creature deals combat damage to a player, that players loses half of his or her life, rounded up.
Voltron strategies, those that involve loading up one creature with a bunch of Equipments and/or Auras, are extremely popular in Commander. This would explain why a great many Kemba, Kha Regent decks have included this equipment. A card like Quietus Spike is especially good on a Commander like Rafiq of the Many, who much of the time has double strike. That means the effect of Quietus Spike activates twice, which means a ton of life lost.
Quietus Spike is also fun on any creature that has trample or otherwise gains trample. This is because when you assign damage on a trampling creature, only 1 of that damage has to be dealt to a defending creature. Now, not only can you deal more damage, but greatly sap your opponent's overall total, as well.
Sadly, the Spike's ability doesn't trigger Gisela, Blade of Goldnight's double damage dealing ability. This is since the Spike's ability is life loss, not damage. The Spike also doesn't grant you additional combat damage when totaling the 21 Commander damage (also known as general damage) necessary for an invididual Commander to take out a player. It also doesn't double infect damage, which is a good thing, because 10 Poison Counters to kill a player is a low enough total to begin with.
The Spike was also quite good in the short-lived but massively popular Tiny Leaders format, the 50-card Commander variant in which you can only play cards with converted mana cost 3 or less. With how prevalent Voltron strategies are in that format, and the lack of lethal Commander damage, having an Equipment like this can end games quickly. Your opponent only starts with 25 life in Tiny Leaders, so it won't take much for any creature equipped with this to land a heavy blow.
The Tiny Leaders format didn't last all that long, but a lot of people bought Quietus Spike that didn't have use for them before. The price has been on an even steadier rise since then. It took a big jump from $3 to almost $4 between February and April of 2016 and the price has been wavering between $3 and $4 since then.
Even without Tiny Leaders, Quietus Spike sees play in a wide variety of Commander decks. Interestingly, the Commander who utilizes it the most is not one with Double Strike. It's actually Olivia Voldaren. Olivia is usually a Vampire Tribal Commander who is definitely focused on life loss, so you could see the Spike fitting into that strategy quite well. Another Commander bent on life loss is Vela the Night-Clad, so it's little surprise that the Quietus Spike shows up in some of her decks, as well.
Also, both of these decks have access to an Enchantment called Wound Reflection. This card has a player-killing combo with Quietus Spike, since it causes each opponent to lose life equal to the amount of life lost that turn at the end of each turn. It's probably the best combo available with Quietus Spike.
Quietus Spike is a fairly valuable equipment in a wide range of Commander decks. While it hasn't become a staple in the format, it's widely played. You can't really go wrong picking this card up if you're fond of Equipment-based decks.