is nosferatu the one that ignores res? i recall that being of only limited usefulness on most druid's unless you spent time raising sophia. however therein lies the problem with speedruns and a scoring system. theres no consideration for characters used. should a player that beats map A in 5 turns with haar get the same score as someone who beats it in 5 with Jill?
No, its the one that drains life from damage. Otherwise known as Nosfertanking where you can stick a Dark Mage/Druid on the front lines without worry since any hits they take is going to be healed back from the damage they deal so long as they don't get OHKO'd in the process. Normally, its not a terrible problem since such weapons are difficult to come by and your Druid is likely only sporting 10-12 Def by their endgame averages while their Spd and Luk aren't entirely enough to avoid getting doubled/critted while Weight tends to be an issue as well (there's also a sword variant known as the Rune Sword and is pretty beastly since its not limited to dark magic users only but tend to be extremely rare as a result).
What makes the tome and its upgraded variant so gamebreaking this time is that a) you get more than one Dark Magic user, b) the tomes are plentiful, c) stat caps are so ridiculously high that even crits tend to be a non factor for most of the time, d) the revised doubling formula gives otherwise slow classes such as Dark Mages and Sorcerers more leeway (takes 5 Spd now to double or be doubled), e) no weapon weight at all, f) you can forge and upgrade the power of the spell among other things making it even more effective at draining life, and finally g) there's a skill that prevents weapon durability loss based on luck*2 and all promoted classes have a default Luk cap of 45 (not including personal cap modifiers or the skill that raises all stat caps by 10 save HP and Move) making making any weapon unlimited use for those who have said skill, and for hilarity's sake h) you can stack the Sol skill on top of that (proccing that makes the hit drain life) for even more unkillable fun.
Of course, the trick with those last two points is that you need to have the correct class combination to both learn those skills and use dark magic (only Dark Mages and Sorcerers can use dark magic and no other magic using class barring the use of a very specific skill which only shows up online or during the postgame). But then, you're guaranteed at least four people who can pull this combination off regardless as to who you sleep with (and much much more if you ever go online).