I am aware that my lore is very flawed and patchy so any input or corrections would be fabulous!
What is their relationship with Frem?
- As has already been mentioned, it is where the powers stemmed from.
What is the cause of this phenomenon (seers)?
- This came as a result of living within a close proximity of the Stone of Jas (while it was on Lunar Island). This awakened within some the power to 'See'. As we have learned the Anima Mundi exists in all life -so in a sense all beings are innately magical. But for some races these powers are dormant -so I'm suggesting that the stone awoke it within the Frems. This could also help explain the recent decline in born-seers. Perhaps the Anima Mundi within them has gradually 'unawakened' from generation to generation (degradation). If this doesn't stay true to the Anima's lore then we can just say it was the stone of Jas.
Why did they resettle in a different location?
- One possibility is that a family of Frem seers (Farsight or some cheesy name like that) grew weary of the perpetual war between themselves and Moonclan and sought out a more peaceful existence. This would also explain their conversion to Guthix's beliefs.
- Another possibility is that this family poorly interpreted a portent that cost the lives of many Frems in a battle against the Moonclan. Dishonored, the were banished and settled a village -aka Seer' village.
-They could have been V's men who helped him build the element altars. The Frems though only gods should make runes so they might not have been allowed to return so they settled the village. See the elemental workshop for more speculation!