The entrance for Altar of Sienna is located in Mutant Forest (MF) right where the MF Grocer is located.  Once you have the entry item in your inventory, you will see a warp.  When you are ready go inside it.

When you warp in you will instantly see a tiny little person.  Click on the tiny little person to activate quest.
Tip: Before clicking on the tiny little person, make sure your party is fully buffed and in bm2 and aura with trans.

After you activate the quest, Glutto will appear.  Defeat him!
Tip: Our team went "Bankai" (BM2 and Aura with trans) and debuff–panic cry, field of fear, hardluck, etc

Once you defeat Glutto, continue on.  You will reach a warp, go inside it.  It will warp you to the next room.  Continue straight and you will see another tiny little person.  Click on him to start the quest.

After clicking on the tiny person, go back the opposite way you came from and make a left.  After you make a left, look for this chest.  Once you found it, click on it.

After clicking on the chest, go back to the tiny person and on his right you will see this tomb which is the Ominous Altar you are looking for.  Click it.

After clicking on the Ominous Altar, go to that same tiny little person and click on him to finish the quest.

Once you’re done and have recieved your gem, continue along the path.  You will see some stairs, go up it and you will see the boss named Zealru.  Defeat him!
Tip:  My team went bm2 first and extended later on.  The reason why is because this boss, Zealru, teleports.  So dont waste your full power on him because he will teleport alot.

After you defeated Zealru, continue forward.  Go down the stairs and proceed through all the mobs.  Once you get to the end, you will see another tiny little person.  Before clicking on him, make sure your team is fully buffed and in bm2 and aura with trans.  Activate quest when your team is ready. 

Defeat Angelo! Be quick when trying to kill this boss, if you are too slow he will "1 hit you" in addition he can go Art of Defense (AOD) for 5mins.  Please note all bosses can go AOD.
Tip: I had the tankers in front of him to aggro, and the rangers directly behind him with good distance.  After your full power is out, retreat directly behind him and hug the wall.  Wait for BM2 again and attack together.  Tankers take one side and rangers take the other, do not attack together in the same direction.

After you defeat Angelo, continue on to a room where you see four of these statues.  On the left side, as you come in, click on this statue (Effigy of Jeolousy) first.

Then click on the one next statue (Statue of Voracity) to it.

Then go click on the first statue (Effigy of Jeolousy) one again.

Afterwards, directly across it, click on that statue (Statue of Wrath).

Now, once you click on the statue head back out and find this red ball (Shining Sid).  Once you find it, click on it. 
Tip: As you are leaving that small room with the statues, make a left turn and you should see it around there.

After clicking on the red ball, continue back the diretion you came from and keep going till you reach a dead end.  You will see this wall, click on it.

Then going back the direction you came from, near the room where the statues are at, find this Brazier and click on it.

After click on the Brazier, go to the red ball (Shining Sid) once again and click on it.

Now then, go back to the statue room and click on the final statue (Statue of Greed) to activate quest.  Once you activate it, you will have to defeat 2 bosses.  One of the left and one of the right.
Tip: Before clicking on the final statue, make sure everyone is buffed and in bm2.  Immediately after you activate quest, run up the stairs and pass the boss on the right side.  You want to pass him and go to the other room.  In that room, go full trans and kill the tree boss.

Defeat Tree Boss!  (This image is not the image of the tree boss, this is of Greedir, who is the other one.  I apparently did not take a picture of the tree boss, I apologize.)
Tip: If your BM2 alone cannot defeat him, extend with aura.  After you defeat him back out of the room and wait for BM2 again.  Again make sure party is buffed.

After defeating the Tree Boss, defeat Greedir!  Defeat him quickly or he will AOD for 5 mins.
Tip: Go full power again, BM2 and Aura with trans and full debuffs.  Make sure tankers are attacking him from the behind, and rangers at the front.

After defeating Greedir, continue on.  You will come to a warp.  Go inside it and it will warp you to the next room.  Follow it all the way through and you will come to this statue (Contaminated Waterway).  Click on the statue to activate the quest.

Now head backwards to the warp you entered in from and click on this Old Altar.

After clicking on the altar, head back to the statue (Contaminated Waterway) and click it again to activate quest.  Warning alot of mobs will appear!
Tip: Before clicking on it make sure everyone is buffed and go Bm2 or Bm1 whichever you prefer for Area of Effect.  Once activated kill all the mobs.  Afterwards, continue forward into the pool to fight the "fish boss, Lazine".

Defeat Lazine! Kill him quick, if your team is too slow, he will AOD for 5mins.

After you defeated Lazine, continue forward and immediately on your left, as you exit out of the pool, you will see a statue.  When you are ready click on it to activate quest for final boss.
Tip: Have your team go BM2 when you activate it and when you get to the final boss, Centrion Prideus, extend and trans with full debuffs.

Defeat Centrion Prideus!

After you defeat Centrion Prideus, take a break with your guild and enjoy the good life.

After your done relaxing, continue on and finish the quest by clicking on the red ball (Gem Sid).