Their powers are exceptionally potent and perilous.
In Marvel comics, some characters manipulate reality. For instance, Franklin Richards, Mad Jim Jaspers, and Scarlet Witch.
However, in Marvel movies, few characters possess the ability to warp reality, and if they do, it's often through the power of the Reality Stone.
So, who are the Marvel characters capable of manipulating reality in the films? Below are three prominent names among them!
1. Thanos (when he wields the Infinity Gauntlet)

With the Infinity Gauntlet, Thanos could manipulate reality itself.
A clear example is Knowhere, where the Mad Titan preserved the place when the Guardians of the Galaxy appeared. Later, it was revealed that the place had been destroyed. Even the Thanos killed by Gamora was just an illusion.
Or the moment on Titan when Thanos recounted the fate of the planet, it was just an illusion. The Reality Stone was the key to all of this.
Essentially, the Reality Stone can warp reality at the wielder's will, turning their thoughts into reality. It can create various weapons, even crafting illusions so convincing that others would believe them to be real.
2. Ultron (What If? version)

In the What If? episode 8, Thanos arrives on Earth, which Ultron has already devastated. Ultron then swiftly kills Thanos and takes all the stones he had gathered.
Ultron seems to harness the power of the Infinity Stone to create more Ultron Sentries, even crafting his own spacecraft. You can see the distinctive red hue of the Reality Stone as he does so.
3. Scarlet Witch

If Thanos and Ultron appear to only manipulate reality when they possess the Reality Stone, Scarlet Witch can manipulate reality without borrowing its power.
In the MCU, Wanda can manipulate and warp reality to a certain extent, as demonstrated in the WandaVision series.
Specifically, WandaVision introduces us to a fictional town called Westview. Stricken with grief, Wanda enters a virtual reality and resides there with Vision. It's a reality full of mysteries and everything seems to be under Wanda's control. Even Vision and Wanda's children are creations of her magical power of 'reality warping' alone.
In the MCU, Wanda is also a master of illusions, capable of deceiving others with incredibly lifelike and vivid illusions. In the opening sequence of Doctor Strange in the Multiverse of Madness, Wanda even deceives Strange with a tranquil garden scene, catching him off guard and revealing all the secrets related to America Chavez and the multiverse.