The contract with hell is at the end of the Ascold quest (or also called quest of the mummy powder).
1°/
the mummy powder : first you have to launch the mummy powder quest (you have to talk with some merchants in store and ask for job. one of them will give you this quest)
find the mummy powder by talking to all the street merchants (you have asked to each one 3 or 4 time the same question : do you have mummy powder ? you can also, after asking this question to some merchant, go back to see the merchant who give you this quest he will say something to you (the street merchant have fear of the inquisition so you have to insist), maybe it's important and maybe you can't found the powder if you don't go back in his store... maybe not. personally I always go back to see him and generally I found the mummy powder a few time after this visit (Curacao look like to be a god place to find the powder)
take the powder back to the merchant, he will say to you to take it to Ascold in Guadeloupe (look in the houses of Basse-Terre to find him).
2°/
the 3 Aztec items : After giving him the powder he will give you a new mission : find 3 Aztecs items.
personally I never found these items in the tresors quest. I find them in Tenochtitlan but only when it was to late. You can buy them in Jamaica. You have to talk to the Henry Morgan's secretary. It will cost 1000000. Take back these items to Ascold and he will give you a new quest (you have to come back after a few days).
3°/
The Spanish list : You have to find the Spanish list of Aztec artefact : go to Havana, talk to the bartender ("i have another thing to do"). after that, go, through the jungle, to Santiago. you will find in a house a Spanish officer. talk to him and he will say to you where you have to go to find the manowar who have the list : the Amatike bay (near Belize). Go back to Havana and to your ship and let's go to the Amatike bay (if you can, because all this part of the quest is very hard).
You will find 10 manowar in the Amatike bay. you have to board therm and to find the list in the desk of the captain cabin. When you find it, sunk the other manowar and go ashore of the Amatike bay. you have to go to the Inca temple, kill all the Spanish soldiers around the temple. After that go in and kill everybody (not easy, you can lose some officers). Don't miss the Aztec totem, a new one who give a bonus to the talent "repair", inside this temple. You can used him in Tenochtitlan to open a desk. In this desk you will find rare Aztec item but they don't have any utility in these quest. You will find in this chest the 3 items you looked for in the part 2. You can go (if you have completed the Tenochtitlan quest orf course. if not you can't) or not, it's not an obligation.
So after killing every Spanish soldier around and in the Inca temple you can go back to see Ascold. You will have a new mission.
4°/
The tomb : Finding the tomb is very easy... go to the cavern of Guadeloupe. When you will be near the tomb entry in the cavern your player will say something. Go back to Ascold and he will asked you to find the tablet. This tablet is needed to open the passage to the tomb in the cavern (at this part of the quest you will don't have any more informations about it in your quest log, so read well the dialogue and use your memory).
5°/
The Aztec tablet : the tablet is in a deserted island (random place). Personally in found it not far of Guadeloupe, in the cavern of Dominique in front of the entry (it's look like an Aztec totem but it's a little bigger). If you don't find it there you have to look in other place like Cayman, Turks etc. when you find it take back the tablet to Ascold and you will have now the funny part of this quest. But TAKE CARE, I've seen a very bad bug in this part of the quest : I lose some officer, my warrior. so remove them from their post before.
6°/
Leif Ericson : After taking back the tablet to Ascold go back in the cavern and put the tablet in the symbol on the floor. You will have a loading screen and then the passage is open. go near the tomb and try to open it. You can't open it of course but you have to try before to go back to see Ascold, he will give you an item to open the tomb (you have to make a lot of travel between the city and the cavern in this part of the quest). So now you can open the tomb. 3 skeletons appears, kill them all. after that Leif Ericson appear and talk to you. he will say to you his story (very funny) after some little tricks (you have a funny surprise there) he will give you a mission : invade Basse-Terre with an army of skeleton, and find 4 items (3 toltecs skull and an another item. You have to speak to Leif Ericson to be able to go out of the cavern and to launch the attack..
You will have french soldiers just outside and in the town, kill them all and after that talk to the governor in the residence. he will give you the key to open the cupboard where you will find the 3 skulls and the other items. Take all that and go back to see Leif Ericson. after a short talk you're free.
Go back to Ascold, after a short talk you realize that you have to save the world... amazing... go back to the cavern and kill Leif Ericson. Take the items he have (you will never seen back the 3 skulls and the another items

). You will find a Vase which is very important so don't forget it (he have also a nice heavy sword).
Go back to Ascold and now you will have 2 options : sell him the vase for 50000gold (not interesting) or to keep it and try to open by your own way. You should keep it.
7°/
The contract with hell : You have to find a way to open the vase. Go in the shipyard and asked to the owner to do it. generally he will refuse and ask you to give it to the church. if you choose to give the vase to the church you will gain reputation (a lot of reputation). If you want to open the vase you have to go in several shipyard. In one of them the owner will said to you to take his tools on a table and to do it yourself (personally it was in Saint Domingue). So open it and you will be teleported in Guadeloupe at the mornl'o shore. A demon (he don't look like) will talk to you and he will propose you different thing (after a long speak, but it's fun to read it). you can ask for everything he will purpose to you. He will give you : money (you have to go in a bank, for me it was in Charlestwon but maybe it's in an random one, and you will have 3000000 for free), rum (in an random store i think you will have free rum, for me it was in Port Royal), better attribute (in one brothel, the owner will asked you to choose 1 attribute and after a little operation you will gain 1 point to it. For me it was in Tortuga but maybe it's an random place), and finally he can give you invulnerability (immunity to all attacks, and i let you read the contract to see the meaning of that).
So now you have a contract for this last pointed. But just an advice : rise the pirate flag in a shore and go, through the jungle, in the nearest town. You are invulnerable now so don't worry. Kill all the guard, they always appear but that's good for you. Kill as much you can. You can go to the fort of this city to. well take fun and kill as much you can. It's also a good way to rise some talent like heavy weapon, pistol, defence etc...after that you will catch enough energy to paid your invulnerability for several day (because you have to pay it with lives) so no problem if you want to make a long travel in sea. of course you can repeat this operation in all the city when you need it (but you relation with the nation will be... very bad of course). If you can't paid for you invulnerability, you loose it. Check your quest log to see how much you have to pay it and to see how much reserve you have.
and that's all for the Aslcold quest. Now you know how to have a contract with hell, how to find Ericson etc

a very nice, and very funny quest. but to hard some time ( the part 3 is hardcore)
edit 1 : sorry for my English
edit 2 : do you Laugh at me Giocoso

k ??? if not thanks