Halliday & Matthiessen (2014: 497):
There are no embedded clauses corresponding to the paratactic and hypotactic categories of addition, replacement and alternation (and, instead, except, or). The only sense of extension which produces embedded clauses is that of possession, introduced by whose, of which/which … of or a ‘contact’ relative ending with of …
In one of those cities – one [[ + whose name has long been forgotten]] – there lived an old halac uinic, or chief.
Note that here, as elsewhere in the grammar, possession is generalised possession; it includes not only concrete ownership but various kinds of concrete and abstract association.