The Sentinel is a being who exists on The Path of Chaos, an extension of The Crossroad of Trials. He serves the Path and appears as a man wearing a dark purple and black medieval suit of armor with his face concealed beneath a helmet. When a person reaches the terminus of The Path of Chaos, he will sometimes appear to reward them.
The Paradox Arc
The Sentinel first appeared in Daniel & Friends, in which he gave two pieces of a key to Tony Helioson and Daniel Ferris on their respective trips. Once completed, the key caused various alternate versions of Daniel from parallel universes to be pulled into The Daniel-Verse. He appeared to collect Red Daniel's body after his death, and it was later confirmed that he collected the bodies of the other Daniels as well. It was eventually revealed that he was yet another version of Daniel Ferris from an alternate universe where he died on the Path of Chaos and was revived as its servant. He has been using the Path to travel to nearby universes in order to absorb alternate versions of himself, gaining their powers. The Sentinel has done this multiple times in the past, and delivered the key to Daniel as part of an elaborate plan to absorb multiple Daniels at once. He succeeded in absorbing Red Daniel, Enhanced Daniel, and Dark Daniel, but was ultimately defeated, and killed, by the normal Daniel.
He is able to move freely through the Path of Chaos and to the 'normal' world. He is able to erase all evidence of a fight somehow. He is also a skilled swordsman.
His primary ability is to absorb other versions of himself and gain their powers. He has used this power to absorb an unknown number of his alternate selves, including Red, Enhanced, and Dark Daniel.
When he is killed, the Path of Chaos was able to gain complete control of his body, transforming him into a four armed monster wielding a larger version of his own sword, the Omnibolg, Dark Daniel's katana, and Red Daniel's crystal blood sword.