1. Talk to Thomas the keeper. Withdraw the soul you wish to use . (like an unknown soldier's soul or the boss demon soul) if you have more than 1 qty of it in storage, make sure you withdraw all.
2. Ready the item or spell in your hotkey wherein you can easily select or use it. The nexial binding will use all the souls you have, nevertheless the splinter will be used up. Just keep it ready this time.
3. Talk to Blacksmith Boldwin. Try buying an item like a crescent moon grass (that's why you need atleast 150 souls to buy the item) Do not buy it. But leave it in the dialog box where you are selecting the amount you wish to buy.
4. Slowly, walk away from Blacksmith Boldwin. the selection screen will close.Walk towards Thomas the Keeper.(Thomas the generous I suppose at this time =) ). Do not walk too fast. If you do, Boldwin will say something like you're being rude, this will interrupt what you are supposed to do next.
5. Try using the spell or item(the evacuate, the ring or the splinter of keystone). There will be selection asking you if you wish to teleport back to nexus. press start to hide the selection.
6. You can now talk to Thomas. Store the soul item you wish to use later. While the dialog screen is still on, your character will cast teleport. the entire game will reload.
7. Surprised? you are still in the nexus. Go talk back to Thomas. Try withdrawing the item you stored.
8. Amazingly, you now have 1024 of the item you store (the soul) You can now withdraw any amount from the storage for use. Withdrawing any amount from the storage will revert the quantity in storage to 99 but you will still get the number of soul item you withdraw.
9. Put the soul item in the hotkey (for use) a Legendary Hero's Soul gives like 8K souls per use. 100 of that will give you 800K souls. You can have as much as 1M soul in 5 minutes!!!!
For faster Soul gain I Used (or duped rather) The Demon Dragon God's Soul which Gives like 24k souls. The only drawback when using a demon boss soul is that you cannot put it in the hotkey unlike the regular souls. You need to open menu by pressing start and selecting the item for use. But it's worth it. one use grants tremendous amount of souls. Re-arranging the item list for convenience will make the process faster. You can net up to 6M Souls (that's right) in 5 minutes. in 15 min time, i was like lvl 106. in 3 hours you can be Soul Lvl 300+ depending on your patience.
By discovering this, you now know too the game-breaking glitch: you can dupe almost any item. Splinter stones, Stones of Ephemeral Eyes, Spices, Grass (Weeds as I call it =P) even the rare Colorless Demon Soul that you can only get like 10 in qty per playthrough.
( I do not know if this still works in updated game version, I used the original, there may be some patches but trust me, this works the best and saves you all the time in the world)