Are you playing 1.3.5.3 or are you playing 1.4.0.5, if you are on 1.4.0.5, its the terragrim that is obtainable from sword shrines instead of the arkhalis
also there isn't a specifc seed which gives the terragrim/arkhalis but instead there is a 2% chance of a enchanted sword shrine giving you the terragrim/arkhalis instead of
the enchanted sword when breaking it (The best way to get the terragrim/arkhalis is to make multiple worlds, search for enchanted sword shrines and break the shrines, repeat these steps until you get the terragrim/arkhalis)
If you are trying to get the arkhalis itself (and are playing on 1.4.0.5), simply open a bunch of treasure bags (only ones dropped by hardmode bosses except for Queen Slime), it is obtained alongside the arkhalis set (this may take alot of time, but I recommend farming Golem for the treasure bags in expert/master mode)