The greatest treasure possessed by each of the Kua Toa monasteries are their cache of Deathfin Scrolls. Only monitors who have been granted the highest rank - Deathfin - are given access to this ancient knowledge, some dating back to ancient times long before all the other races of the sea had been subjugated.
The restriction on who can access these 'scrolls' is not only for secrecy. The techniques are all considered extremely difficult and powerful, but are almost always considered dangerous even to learn. It is said that the oldest monasteries (Diamond Reef and the Sons of Baal) originally tried to share their greatest warriors' secrets with ordinary Diamondfins and Finlords, but that too many monitors died when trying to train their bodies, minds and chi, until the Godfins decided that only a ful master of their arts - a Deathfin - would have both the power to make use of one or more scrolls, and the wisdom to abandon a technique they could not master before being destroyed by it.
Since the restriction was introduced, most Deathfin monitors now spend years searching through the scrolls of their order, trying to find one or more of these techniques which fit their own personal style, body or mindset, and then attempt the often insane training required to master a scroll's secret arts.
Few examples of Deathfin Scrolls are known outside of their own monastery (though each monastery is always trying to learn what secrets their competing orders possess), but Blood Mountain is known to have the infamous Molten Scroll. The Molten Scroll may contain more than one technique, and several Deathfin monitors have actually died attempting to master it, while others who have tried have wisely abandoned it. But this scroll is known for the Touch of Molten Death, a much feared attack which according to legend, "no living thing can survive". Godfin Shuazul is in fact said to have survived the Touch during the Shadowfin Heresy, but historians (and Monitor fanboys) debate whether this means the Molten Scroll's deadly reputation is overblown, or whether "Shuazul doesn't count" (an often repeated argument in such circles).
While each of the existing monasteries protects the vault containing their scrolls with a mix of secrecy and/or traps and/or magic, of the two defunct monasteries, the Silver Shoal Scrolls (aka the Silver Scrolls) are said to be lost, while the Shadowfin Scrolls were destroyed at the end of Shuazul's Heresy by the surviving Godfins, by order of the Emperor. Unless you believe the conspiracy theories...