Things we know as canon:

• Gravesfield was founded in 1635 and Philip's journal dictates his arrival to the Boiling Isles in the 1600's
• There are two statues in the Gravesfield town center depicting the likeness to Philip (via the echo mouse journal videos) and another man.
• Two brothers were "lured" by a witch into a demon realm never to be seen again. One brother is holding "glyphs" and the other showing a bird on his shoulder. The townhall also appears to have burned down.
• Philip was exploring how to create a portal from the demon realm to the human realm, and discovered fools blood. His traveling partners weren't as lucky as he was in escaping.
• We see "grimwalker" instructions/recipe amongst Belos's things
• Almost all the titan's blood is used up
• Witches can lengthen their lives by some means
• Alex Hirsch works on the show

Theory:

Philip was exploring wild magic in the human realm and followed a witch into the demon realm to learn more. The townhall burned the door portal into the demon realm after Philip's brother (Belos) followed them leaving them both stranded away from home. Each brother had a very different experience on the Boiling Isles which lead Philip to discovering his innate magical abilities while his brother was probably hunted/tortured/scarred. They eventually found each other when they come across fools blood in eclipse lake. Philip escapes the explosion and assumes his brother perishes (but he is alive). Assuming his brother is dead, Philip found a way home and learned that magic doesn't work in the human realm, so he returned to a quiet life mourning his losses never revealing himself or his brother's "death". The townspeople erected statues in their honor because they assumed both dead/missing. His brother however, assumed Philip abandoned him and held a grudge against wild magic due to this and his trauma on the isles. He found a way to prolong his life determined to remove magic from the isles and return to the human world once and for all. With each prolonging he became less and less human, leaving him the way we see him without his mask, bruised old and deformed. Since all the titan's blood is used up, he now needs another very powerful magic source to return home (all the coven witches magic will be drained to do this).

Luz will have made her palisman by then (most likely a snake mimicking the sigil for healing). Eda will continue to live with the curse (having finally accepted it) and they will "save the day", but not before destroying the portal back to the human realm leaving Belos AND Luz stranded in the demon realm (again). Luz will be able to stay in the demon realm without breaking her promise to her mother (if she came home she would stay).

We know Alex loves using twins/siblings and their separation in shows (i.e. Gravity Falls. This is a strong source for why I think Philip and Belos are twins (or at least brothers) and were separated.