Isildur's Bane

[CSteefel]

Gandalf specifically says that Denethor apparently did not know the whole story of the Ring and he guessed that the scrolls of Isildur were read only by himself and Saruman:

And Boromir, there lies in Minas Tirith still, unread, I guess, by any save Saruman and myself since the kings failed, a scroll that Isildur made himself.

The verse about the Halfling and the discovery of Isildur's Bane was known to all, as was the fact that Isildur did not go straight to Arnor, but planted the White Tree in memory of his brother, but Gandalf says:

"'But in that time also he made this scroll', said Gandalf; 'and that is not remembered in Gondor, it would seem. For this scroll concerns the Ring..."

Early on, few knew of the existence of the Ring at all (with the exception of the Wise) and of those, most thought it was lost per the suggestions of Saruman. The focus in the first meeting with Denethor is on the possibility that Aragorn will be coming to claim the throne. However, in the second meeting upon the return of Faramir, Denethor seems to already know something of the meeting between Faramir and Frodo, and then states in referring to what Boromir would have done

He would have brought me a might gift.

That this is something like the Ring is explicitly acknowledged by Gandalf

'Comfort yourself!' said Gandalf. 'In no case would Boromir have brought it to you. He is dead, and died well; may he sleep in peace! Yet you deceive yoursef. He would have stretched out his hand to this thing, and taking it he would have fallen. He would have kept it for his own, and when he return you would not have known your son.'

This is followed by Denethor's response

'But I who was his father say that he would have brought it to me...I have in this matter more lore and wisdom than you deem.'

or even more explicitly

...would he have set this thing at hazard beyond all but a fool's hope, risking our uttter ruin, if the Enemy should recover what he lost.

which indicates pretty clearly that Denethor knew this was the Ring that Sauron had lost.

So it still is not clear whether he read the mind of Faramir, who figured out that this was the One Ring, or whether he knew it much earlier, in which case Gandalf seems to be mistaken about what Denethor knew early on. My supposition would be that he did not know the story of the Ring when he sent Boromir to Rivendell, but that he deduced it during later events, perhaps using the Palantir to read the mind of Faramir, since there is explicit reference to Denethor knowing much of what passed between Faramir and Frodo...

[Elladan and Elrohir]

I always got the impression that Faramir merely told about the Ring when he detailed his conversations with Frodo before his father and Gandalf. But maybe he didn't. I can't say, not having the book in front of me. I need a key chain that has a miniature copy of LOTR on it!

[CSteefel]

It's possible that this is true. It is said that Faramir "told his tale..." So perhaps this did include the details of the Ring, but maybe not, since Denethor shortly says

I can see and hear, as was my wont; and little of what you have half said or left unsaid is now hidden from me. I know the answer to many riddles.

Presumably he is referring to the riddle about the discovery of Isildur's Bane and the Halfling, but it still isn't 100% clear whether Denethor knew this all along, or whether he guessed the riddle then. In any case, there is the implication that Faramir did not tell the whole story, or there would be nothing left unsaid...

[Boromir88]

What's interesting about Isildur's Bane, is that it doesn't seem like the Gondorians at this time understood it was The One Ring. Legate gives Faramir's answer to Frodo and it always looked to me like Faramir attributed Isildur's Bane to the Orc arrows that killed him...then later upon finding out Frodo had Sauron's Ring he surmises that this was Isildur's bane. However, from the quote that Legate gives, it certainly appears that Faramir at first thought Isildur's Bane was the Orc arrows that killed him, then through some consideration he reaches the conclusion that it was actually the Ring.

What Boromir tells the Council about possibly what Denethor knew:

Seek for the Sword that was broken:
In Imladris it dwells;
There shall be counsels taken
Stronger than Morgul-spells.
There shall be shown a great token
That Doom is near at hand,
For Isildur's Bane shall waken,
And the Halfling forth shall stand.

Of these words we could understand little, and we spoke to our father, Denethor, Lord of Minas Tirith, wise in the lore of Gondor. This only would he say, that Imladris was of old the name among the Elves of a far northern dale, where Elrond the Halfelven dwelt, greatest of lore-masters.~The Council of Elrond

Denethor only revealed the 'riddle' about where Imladris was at, that doesn't mean he didn't know more, that's just all he chose to tell his sons. Denethor I would think had actually guessed more considering him reluctantly letting Boromir go on this trek to Rivendell:

Therefore my brother, seeing how desperate was our need, was eager to heed the dream and seek for Imladris; but since the way was full of doubt and danger, I took the journey upon myself. Loth was my father to give me leave,...~ibid

Denethor is very hesistant about Boromir going, but since Boromir (according to Faramir would have it no other way) Denethor finally gave in. I think it's safe to say that Denethor had some worries about this riddle and this trek to Rivendell as he was very hesistant about letting Boromir go.

Basically I'm saying that Denethor probably knew more than what he had told his sons. Or at least this riddle of 'Isildur's Bane' began to trouble him and he went further to investigate. Denethor definitely had the access to the records in Gondor, he even had access to records that no one else did, and only the Steward could view...for example all the lore in Gondor concerning the palantiri. Where if I remember correctly (from Unfinished Tales; The palantiri) only the Steward and the Steward's heir had access to these. Denethor definitely had access to all the 'secret' accounts hidden from the rest of the Gondorian public. Boromir says he's Gondor's greatest Loremaster, and he and Faramir actually went to their father to try to decipher this riddle. Denethor is no chump and no oblivious idiot when it comes to matters concerning his Kingdom. Isildur's Bane would definitely be a matter concerning his Kingdom, and I think Denethor would be aware of this...he may have had some guesses and thoughts before, but I'm sure he ended up figuring out the answers...some descriptions of Denethor:

"...nor had he [Sauron] any servant whose mental powers were superior to Saruman's or even Denethor's."~Unfinished Tales; The Palantiri

In this way [viewing the palantir] Denethor gained his great knowledge of things that passed in his realm, and far beyond his borders, at which men marvelled; but he bough the knowledge dearly, being aged before his time by his contest with the will of Sauron.'~Appendix A: Gondor and the Heirs of Anarion; The Stewards

When it came to matters concering with Gondor Denethor was no oblivious fool. If it something that concerned Gondor, he was going to eventually find out about it. I don't know whether Denethor knew at first what Isildur's Bane really was, he may of had some guesses, but I don't think there's doubt he soon found out about it being the Ring. As Denethor had access to all these records, and when it concerned Gondor he wanted as much information as he could get.

[CSteefel]

I am still not convinced Denethor knew the whole story of the Ring when he sent Boromir off to Rivendell. It seems to me that there is abundant evidence that he figured things out along the way. And I don't see that Faramir told him everything about his meeting with Frodo, since Denethor makes specific mention of the "half said" and "unsaid" things. But one cannot rule out that his prior knowledge allowed him to interpret Faramir's story correctly. Or, he was able to make use of the Palantir to gain knowledge of the Ring as events unfolded.

This last point leads me to a suggestion, which is that the Ring actually drove Denethor mad, not the solely the struggles with Sauron using the Palantir and the death of his oldest son. Normally, proximity to the Ring is apparently required for it to corrupt somebody, as in the case of Boromir, or Isildur much earlier. But because of his ability to see events elsewhere with the Palantir, I wonder whether Denethor was able to feel the corrupting pull and power of the Ring. Some of his statements in the chapter The Pyre of Denethor are very similar to those made by Boromir just before he tried to seize the Ring from Frodo.

But the key is still the Palantir, which allowed Denethor to be corrupted and finally driven mad by the Ring from a long distance away. The loss of the Ring as it went into Mordor with Frodo may have been the final blow here, more than anything particular that happened with his sons.