Head all the way back to where you started and Alucard will inform you of where the summoning of Satan is occuring (I'm not sure if the trigger was time based or you needed to be in the room next to the map device). Head back to the surface by following the arrow (several grapple chains and misting through bars). When you emerge, go through the metal bars on the other side straight ahead of you. The path to get there is the same as last time (and pretty simple) so follow the guiding arrow until you reach the courtyard. A large group of dark monks will be waiting. The tactics are the same as before, concentrate on one at a time, don't let them gather and summon a Dark Apostle and get rid of any summoning circles as quickly as you can. AoE attacks can be suprisingly helpfull against these enemies as they can hit the summoned minions, the portals and the monks at the same time.
Continue following the guide arrow through paths we've been along before until you drop down to the road from between the rooftop billboards. A horned demon will be waiting for you on the street below. Afterwards, there is a nearby magic font through the metal bars if you need to replenish your magic. Then continue onwards down the street to find a second horned demon and destroy it as well. Replenish your magic if you need to and follow the newly appeared purple trail. To reach it, you must head to the tunnel underneath where you dropped down to the street from. Mist through the fence and regroup with Alucard. Follow the purple trail into the church (use the nearby grapple chain and double jump for the hole in the wall). Drop down inside and head for the mirror of fate mini game once more. Complete the simple puzzle and open the doorway.
Watch the cut-scene and catch a ride on ba'ul, or the giant blue flying dragon (otherwise known as the Leviathan). You will need to start climbing upwards to reach Satan but of course that isn't going to be easy. Gouts of green gas erupt from the hide of the creature and will damage you if they connect. Once you are high enough, the Leviathan will level out. Head through the energy mine field to the glowing chain at the far end.
Once that is done you will be back to grappling. Move along the side of the beast, continuing to avoid the bursts of energy. Once you reach the back, you will have to face ten dark monks (in groups of four and then two) and defeat them all before you can use the chain again. Once the enemies are dead, yank on the chain After some cut-scene it's time to take on your son (who is being controlled by Satan, natch).
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Boss: Alucard
Attacks
Flame Jump: An unblockable symbol will appear, and he will jump towards you, creating a vertical flaming spin. He will then briefly pause before creating a medium shockwave out from him. Dodge to the side to avoid the jump, and dodge away to avoid the shockwave.
Whirlwind: With his sword behind him, an unblockable symbol will appear. He will then head towards you and spin his sword in a circle around him twice, in two slashes. Dodge away to avoid.
Teleport Slash: He will teleport away in a black cloud. Another cloud with form where he is teleporting to, and an unblockable symbol will show where he is. He will then jump forward and slash horizontally at you. Dodge away to avoid.
Flame Slash: An unblockable symbol will appear, and he will slash his sword down, creating a vertical flaming spin. Dodge to the side to avoid.
Slash Slam: He will slash horizontally twice at you, and then an unblockable symbol will appear as he holds his sword behind him, and then slams it down in your direction. Dodge away or jump to avoid the first slashes, and dodge to the side to avoid the slam. Note: After the slashes, he may finish with another attack, such as Flame Slash.
Slime Wave: He will punch the ground in front of him, creating a puddle of slime around him, and a wave of slime will launch towards you, leaving a trail where it was for a short time. Sometimes before this attack, he will jump away from you. Dodge to the side or jump to avoid.
Tentacle: He will pull his arm back, and launch a tentacle towards you, swipe it to the side towards you, then slam it down in your direction. Sometimes he will only do the first launch, and then pull it back. Dodge to the side to avoid the first, jump to avoid the second, and dodge to the side to avoid the last.
Aoe Slime: He will put his arm to his other side, then pull it back to cause an aoe slime around him. Dodge away to avoid.
Dash Slash: He will put his sword behind him and an unblockable symbol will appear, then he will bring his sword down and dash and slash quickly towards you. Dodge to the side to avoid.
Slime Wall: He hold his arm up, spurting slime from his arm, then punch the ground. Then a wall of slime will launch towards you. This is larger in width and height than Slime Wave, but wont leave a trail. He may do this attack more than once in a row. Dodge to the side to avoid.
Alucard Meteor: Camera control will be taken from you each time he does this attack, showing him jumping up and powering up. Then randomly around the area, targets will randomly appear and he will slam down, one at a time, leaving a puddle of slime after each one for a while. Once he has finished this, the camera will show him again with his sword, and he will lunge down towards you, causing a shockwave around where he lands. Dodge around to avoid the drops, and dodge to the side to avoid the last lunge.
My first piece of advice for this fight is to use you items. This is the last fight in the game (yes you don't directly fight Satan at all. Bit of a letdown) so there is no point in keeping stocked up anymore. Use your increased mastery items to increase your damage and move pool as well as any healing items you have left to make life easier. Alucard is ridiculously fast so you will nearly always need to dodge. He also has the ability to chain attacks. This means he may change attack pattern on the fly so he is more difficult to predict than the previous bosses.
Use your whip to keep back far enough to have reaction time for Alucard's attacks. After Alucard takes a decent amount of damage (about two thirds of a bar) he will start using his black slime attacks. When his bar is fully depleted for the first time, he will jump into the sky and target you. If he connects it will do decent damage so try to avoid it by dodging constantly. Once he returns to the ground the fight will continue as normal. Keep up the pressure and the damage and you should win in far shorter time than it took to beat death.
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Once you have defeated Alucard, watch the cut-scenes and give yourself a round of applause as you have beaten the game (and a very annoying final boss).