- Informational content: "es gibt einen Ort" and: "Kolumbus wollte dahin fahren"
- Relative clause: replace the demonstrative adverb "dahin" with its wh-counterpart (Relativadverb) "wohin" and put it in front of the clause (and observe verb-last for the embedded clause). You get "(ein Ort,) wohin Kolumbus _ fahren wollte"
The same as a relative clause with relative pronouns:: "Ein Mann, ich kenne ihn." - "(ein Mann,) den ich _ kenne"
The w- / d- Alternation can be looked up in https://de.wikipedia.org/wiki/W-Wort
PS: Maybe there is another thing which creates a feeling that the example is difficult: the relative clause depends on a proper name, "Ostasien". In such a combination, there is something of a clash and the relative clause starts behaving as a comment, it's not a restriction. But the grammar of the relative clause is always the same.