Well, when I said 'Long ago', I meant LONG ago. The Lord refers to Karthus as 'A man who decided to join us', which could mean that he saw Karthus when he was still alive as a human. Since we know that the Isles of the past were a quite normal place before the change happened, it would take them a lot of time, maybe several centuries, to transform into what they are now - unchanging grey trees that carry spectral leaves, everything is as if frozen in time perpetually, as described in Karthus' bio. With so much time passing, regardless of circumstances, the Lord would pass away of old age. His presence at the altar is probably the duty that he was assigned; but in general, the whole Isles are now on lockdown, waiting for -something- to happen. Hecarim, Mordekaiser, and others are their first movements in a very, very long time - so long that the Shadow Isles were a legend until Hecarim gloriously dashed through half the continent and into the League.