Obelisks are enormous Necron war machines and are much larger than your standard Monolith. These engines of war specializes in air defense. They are able to remain dormant beneath Tomb Worlds for millions of years; when awakened by their Necron creators, the Obelisk rises from the earth and levels out far above the surface, blasting enemy aircraft out of the sky and melting infantry with an unholy amount of dakka from its Tesla Spheres. For exceptionally larger flyers like say, a Manta, the Obelisk would then use its Gravity Pulse, which is a weapon capable of creating an expanding sphere of earth-shaking gravimetric force that rips apart the giant aircraft out of the sky and throws your $1000+ model into the trash can.
The Living Metal walls of the obelisk are lined with Canoptek Leeches, allowing it to self-repair in battle. Coupled with the fact that Necrodermis is already tough beyond proper reasoning and able to regenerate on itself, this makes the Obelisk not only an annoyance, but a really-tough-to-kill annoyance.
To further better explain its bullshit powers: "The Necron Obelisk as a Super Heavy is as burly to kill as an Imperial Knight, given it's better armor and the potential of Living Metal. Its high vantage point, while making cover rather difficult to gain, gives it superb lines of sight and the actual girth of the Obelisk allows you to really force an enemy unit to have to circumnavigate quite far to get around it. What almost everyone comments on is its very creative ability to ground gravitic types of units like Wave Serpents and Jet Bikes; or winged killers like the winged Nurgle Daemon Prince or the mighty Storm Raven. Every one of these types of units and more must take a Dangerous Terrain test when moving within 18" range of the Obelisk even when they normally wouldn't worry about such things! Over the course of the game, while it certainly is far from a guaranteed killing mechanism, it absolutely impacts enemy movement decisions and it happens on the enemy turn so it's all a bonus. That can be a lot of terrain tests. That can be a lot of suboptimal decisions made by the enemy."
If you are a flyer dedicated player and aren't shitting yourselves yet, then you may or may not have Imperium-issued brass balls. Regardless, you have been WARNED.