As new sets are introduced they come with new mechanics. Some of these stick around and others simply haunt magic’s history. Legends which was released in 1994 had a bit of a hit and miss approach to mechanics. Expanding on Banding, and adding Rampage, whilst also laying the groundwork for the Commander format. As it gave us both Multicoloured and Legendary cards. In order to counterbalance this new creature types they printed the Card of the Week, Arena of the Ancients.
The oracle text can be found here.
Arena of the Ancients is an Artifact for 3 colourless. When it comes it to play it taps all Legendary creatures, and prevents them from untapping in the untap step. This unusual stax piece won’t just hit the commanders but will hit a decent number of creatures in the 99. As Legendary creatures continue to be printed you are increasingly likely to play against them. This can completely shut down decks if they are going for a Voltron approach and need their commander to attack. Or if they have an activated ability that requires them to tap. The low mana value and generic cost means you can get it out early and easily.
However, Arena of the Ancients is a global effect and will effect you as well. Additionally it will not stop Legendary Creatures from untapping outside of the untap step.
Just because Arena of the Ancients can go in every deck, that doesn’t mean it should. It is at it’s strongest in a stax deck such as Grand Arbiter Augustin IV. Or Urza, Lord High Artificer where it can also be tapped for mana. You could run it in a Kalamax, the Stormsire build that is more focused on slinging spells. Ultimately it should be run un decks where tapping to attacking with your commander isn’t part of the plan.