"It was a striking design, akin to an eye in shape. The Beast seemed transfixed by it and terrified at the same time."
Talismans (non-canon name) are objects that serve as obstacles in The Last Guardian. Strategically placed throughout the Nest, they prevent Trico from progressing past certain areas without the Boy's help.
Talismans are bizarre, yet strangely beautiful contraptions that vary in color and design. However, they share a lot of similarities. They are composed of an overall hexagonal/circular frame with stained glass panes. In the center is a design of a large eye with a sclera. Around the center are triangular shapes of varying colors.
Barrier Talismans are large, stationary devices that are either mounted on rails, hung on walls, or suspended from chains. Talisman Shields are about two feet across and are carried by a select few Armored Knights.
The Talisman design is not unlike a very real tactic employed by moths and butterflies in reality, like the Buckeye Butterfly, which has large eyespot patterns on its wings which can startle away predators, not unlike how these devices deter Trico.
Effects on Trico
The Talismans seem to be little more than decorations until Trico comes to within a short distance.
Suddenly, Trico tenses up, freezes in place, and locks eyes with the strange device. His eyes glow pink and he begins to back away. Putting him in some kind of trance, Trico can do nothing but stand there, whining and petrified with fear and unable to flee or destroy the Talisman without the help of the Boy.
It is never explained why Trico is so affected by the Talisman design. Whether it scares him with color, magically putting him in a trance, hypnotizing him or whatever, it fulfills its purpose well.
How to overcome these obstacles
Given that no weapons can really harm the mighty beast, while walls, gates, and rickety bridges only slow him down, these devices prove very effective against Trico in immobilizing him and rendering the beast harmless. However, the objects have absolutely no effect on the Boy, who can approach them and destroy them. Some can simply be pushed out of the way, others have to be broken with a rock, and others yet have to be cut loose from their supports, causing them to shatter. Talisman-wielding knights have to be headbutted in order to make them drop their shields, which gives Trico an opening to wipe them out. Some are completely out of reach of the Boy or Trico but can be destroyed from range with the Mirror.
Also, Trico has shown, at one point, the ability to overcome his fear of the Talismans. When approaching the Citadel, the two are ambushed by a large army of Knights who attempt to steal the Boy away while Trico is blocked by two Talisman barriers. However, fearing for the Boy, he somehow forces himself forwards, smashing the Talismans and a number of Knights. Rescuing the Boy, the two glide away to safety. It becomes apparent at that point that Trico's bond with the Boy allows him to overcome any fear for his own safety, so long as the Boy is safe.
The boy and beast duo encounter their first Talisman shortly before they enter the chamber where Trico enters a trance and swallows the Boy alive. It lies on the ground to the left of a large doorway, and Trico is startled by it. However, it's easily pushed off of the ledge on which it sits.
Some time later, in a corridor, they come across two more Talismans, one hanging on each wall, facing each other. The Boy, in a voiceover, realizes that Trico is terrified of them, although he is not sure why.
As Trico and the Boy progress, they encounter Talismans in increasing numbers. It becomes apparent that they are not oddities, but placed strategically to keep Trico-beasts out of key areas like the Citadel and access points. While it is unknown why these objects affect the beasts in such a manner, it is likely that the Master of the Valley uses them to protect it from the possibility of a rogue Trico.