Actually that thread has no info on how to actually summon Thanatos o.o; It's a walkthrough through the dungeon but stops at the 12th floor where you place the crystals in order to get to the top.
Here's a guide through the last floor of thana.
You will need 4 fragments.
Fragment of Agony, Fragment of Despair, Fragment of Sorrow and Fragment of hatred. All of these fragments are dropped by different Thanatos monsters throughout the tower. All Thanatos monsters are Ghost 4 property so you canot AD or Asura them. It's a good idea to get a Scholar with oyu in the tower with elemental converters. This will make killing the Thana monsters a lot easier.
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Fragment of Agony is dropped by Dolor Of Thanatos on floors 5 and 9
This one has quite a bit of MDEF so physical attacks are recommended. Other then that, it's actually quite easy. Watch out for that JT thingy he does every now and then and you should be okay.
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Fragment of Sorrow is dropped by Maero of Thanatos on floors 6 and 10
This one can cloak. So have sight/Ruwach up constantly since she can attack while being invisible. She also has quite a bit of MDEF so melee attacking will get rid of her.
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Fragment of Despair is dropped by Despero of Thanatos on floors 7 and 11
He has low MDEF and DEF so spell spamming is your best bet. HW spamming SG and/or JT will get rid of this guy pretty fast
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Fragment of Hatred is dropped Odium of Thanatos on Floors 8 and 12
This one uses ranged attacks so get your HP to cast pneuma somewhere and have the tanker move in there. The Odium will keep on attacking the tanker while the rest of your party can get rid of the Odium. Low MDEF but high DEF so use spells.
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Now once you get to the final portal after putting the crystals in the statues, just walk up the stairs, they're pretty long stairs but there are no monsters on this floor so don't worry
Once you get to the top take some screenies go eeat soemthing etc etc. Do anything you want to do now rather then in the middle of the whole thing.
On one of the edges of the tower, on one of the floating platforms you'll find a magic circle (thana_boss 217, 167). For this you will need to have the item "Fragment of Agony" or you will be unable to proceed. Fragment of Agony is dropped by Dolor of Thanatos. Select the magic circle and enter "Fragment of Agony". At this time, a Dolor of Thanatos will appear. You will need to defeat all of these monsters in order to continue. Once you've defeated them there will be an announcement "Magic Circle A has been activated"
Once the first wave of monsters is defeated, proceed to the second platform (thana_boss 202, 75). This time you will need the item "Fragment of Sorrow". Without this item, you will be unable to proceed. Place the Fragment of Sorrow in the magic circle, a Maero of Thanatos will appear.
On the third platform (thana_boss 80, 76), you will need the "Fragment of Despair". When you place the fragment, a Despero of Thanatos will appear.
On the fourth platform (thana_boss 62, 171), place the final stone, the "Fragment of Hatred". With this stone in place, a Odium of Thanatos will appear.
With all four stones in place, you can finally break the seal. On the final platform, there will be a switch (thana_boss 141 218). By activating this, the Thanatos Phantom will appear to fight you. Good Luck!
If you're unsure what fragment to put where then just go on to one of the platforms and talk to the circle. Then type anything you want there. When you type something wrong the circle will not activate but you'll hear "Screams of Agony/Misery/Despair/hatred from which you'll know on what circle you are at the moment. Also be advised that the spelling is important, even capitals.
fragment of sorrow is not the same as Fragment of Sorrow. Type them exactly how you see them here.
