How much do I add to the CR? (looong post)

Hi again,

You guys always give me such useful feedback, I thought I'd toss this
one at you. The scenario: Grimtooth's Dungeon of Doom adventure.
Lots of very deadly traps and monsters. The players volunteered
knowing full well that multiple character deaths will be possible and
even likely. Prizes will be awarded at the end for things like Most
Character Deaths, Most Spectacular Death, etc.

There are 5 players, and they are currently level 8 (but only four
players showed up last game). Each has been awarded a special magic
item beyond their means. Duplicate items are not available, however
(so, only one Ring of Elemental Command (Earth) or Wings of Flying per
party). No wishes.

So, this last session, they again found the hardest/shortest route
(thanks to the Search Monkey character with the Robe of Eyes) and
decided to take it. They triggered a trap on purpose, and were faced
with an Elder Earth Elemental (CR 11) carrying a greatsword. The
greatsword happens to be magical in the following ways:

"There is a stone in the pommel that not only renders the elemental
immune to magical attack, but casts a counter spell trapping the mage
who cast the spell as if by Trap the Soul. This trap will consist of
pentagram etched in magic fire on the floor of the room and will last
only 30 minutes."

"[Once the elemental is slain] the elemental's blade will remain, but
in the hands of a normal delver, it will be only a Huge +1 Greatsword.
The gem in the pommel will still be magical in nature, but it will only
ward off direct magical attack (SR 20), not trap the spell caster."

This is according to the updated 3.5 version of the Dungeon of Doom.
I'm guessing the folks who updated it thought I should treat the weapon
in melee as if it were a Huge +1 Greatsword in the hands of the
elemental as well, however I also own the old AD&D version of the
Dungeon, and this is what it says:

"[The elemental's] great sword will receive 50d6 in combat, and will
have an effect on any magical creature, just like it would on a normal
mortal! A note to all wizards, the stone in the pommel not only
renders the elemental immune to magical attack, but casts a counter
spell trapping the wizard who cast the spell. This trap will consist
of a pentagram etched in magic fire on the floor of the room, and will
last only three turns."

"[Once the elemental is slain] the elemental's blade will remain, but
in the hands of a normal delver it will be only a 18d6 weapon. the gem
in the pommel will still be magical in nature, but it will only ward
off direct magical attack, not trap the spell caster."

In the back of the old book, it says for AD&D:

"The sword, in the elemental's hands, will have +5 to hit and willcause
5d10 points of damage (5-50)."

"In the Elemental's hands, this sword will cause 5d10 points of damage
(5-50). In the hands of mortal beings, however, it will act as a
normal Two Hand Sword so far as damage goes. It does retain some
enchantment, however, giving it the semblance of a +3 sword for the
purpose of hitting magically protected beings and undead, without
imparting any actual weapon plusses."

I chose to make this sword do 50d6 of damage (in the hands of an Elder
Earth Elemental only) for several reasons.
1. The room in which the elemental is in is on the threshold of their
goal. Unless they choose to fight the wizard whose tomb it is (and
they can choose not to without any ill effects), this is the final boss
battle.
2. Since the players have access to overpowered weapons/armor/magic
items, I see no reason for one of my bad guys (and only one, in the
entire dungeon) to have an overpowered item as well.
3. Most importantly in my mind: there is a way for the players to
defeat this elemental without taking any damage to themselves
whatsoever. When nobody else is in the room with the elemental, it
simply sits back on its throne in full view of anyone in the hallway
outside the room. It does not pursue anyone out into the hallway.
They can shoot at it or toss in molotov cocktails or non-targeted magic
(like a fireball not targeting the earth elemental) without stepping
into the room, and the elemental will take damage and do nothing to
defend itself.

So anyway, this group of adventurers had thus far been doing a great
job of finding and bypassing all my traps and monsters with hardly any
issue. I was rather surprised (though perhaps I shouldn't have been)
that they suddenly got careless and decided to just go ahead and set
off a trap that they knew was a trap and with everyone all in the room.
The room was 20x20x20 with a short hallway leading into the room.

When the huge earth elemental with the huge greatsword suddenly showed
up, they all decided to fight it together. Even Search Monkey
uncharacteristically stepped forward in the first round using his Cloak
Dance to "provide another target."

Now, they didn't know how much damage this thing could do, so for its
first attack I had it attack and sunder the cleric's +2 tower shield.
That should have been their cue to retreat. Only one of the characters
retreated, however. The others stayed put. Nobody knew that the
elemental also had Great Cleave, either, so when the cleric failed his
concentration check (he was going to try to dismiss the
elemental...ha!), the elemental killed him in one blow, and then also
killed one of the other characters. The elemental got to go before
Search Monkey (because Search Monkey used his turn to step up to the
elemental), and on its second attack, the elemental killed Search
Monkey.

This effectively eliminated all of the threats within the room, so the
elemental went back and sat on its throne. The last remaining
character observed this, and really wished his fellow party members had
listened to him when he sounded the retreat. Of course, they had no
way of knowing the elemental wasn't going to follow them out, but they
had a pretty good plan even if it did -- they were going to re-trigger
a pit trap while it was standing in the right place. Unfortunately,
nobody listened to the retreating player.

Anyway, there were several ways they could have avoided dying in this
battle, so I don't feel particularly guilty about giving this elemental
such a powerful sword. Another GM I spoke to about it said I was being
too tough, even given the nature of the adventure.

I disagree, and anyway what's done is done, but I'm still left with
this question: how much do I add to the CR of this beast?

I was thinking 3, 2 for the absolute toughness of the damage it can do,
and 1 for the magic abilities of the gem in the pommel. I know what
you're thinking: a CR 14 is way too powerful for 5 level-8 characters,
even if they have some extra powerful items themselves. However, like
I said, they can beat this thing with time and resources without ever
being in danger themselves, so I think it's a reasonably fair exchange.