> is it not recommended to go
>over the "weird stuff happens here" threshold too early in the
>game? i understand that 15+ (?) soft insight is where the
>changes happen

I think there are three insight thresholds, probably at 15/30/45, plus other environmental alterations that happen after defeating certain bosses. For the most part increasing the insight threshold will slightly increase difficulty in various ways (like making you more susceptible to the frenzy debuff), while also enabling higher damage output if you can take advantage of beasthood. Conversely, low or nonexistent insight can make certain encounters hilariously easy.

>unrelated (maybe it is related to insight...): what in the
>world is that time/space rift near the old cathedral that only
>appears as you approach the body near the tree? is that how
>you start the DLC?

Yeah, that's for entering the DLC. And related to insight.