looks like a fungus...if it were mine I'd cut off the effected part because the rotting will continue. I wouldn't water it any more for awhile and I'd put it outside in the shade (good air circulation) where it won't get rained on.
If you see improvement after a couple of weeks give it some water.

Thanks, Laura! Would cutting the leaf half way affect the plant health negatively? I am assuming it'd have to work up the strength to heal the cut on leaf. Just wondering if the whole leaf needs to be removed for better health. The same bunch had another leaf already halved when I bought it.
Also, are there any indicators for early onset of fungus on sans. and prevention/curing techniques?

Please make sure the knife or scissors are clean and wipe with alcohol. It may have picked up a fungus when the sales person cut it. A lot of fungus is spread this way.
Good luck.

Hi,

You might as well cut the damaged leaves in half. I'm sorry to say the damaged areas will not heal, so you might as well cut them off. Hope the rest recovers for you.

What I would do is cut away the bad parts of the leaf, using an X-acto blade or single edge razor blade so you are not cutting by crushing, which would be the case with a scissors. I would make sure the plants are VERY WELL VENTILATED. My best guess is that at some point, something damaged the plant, and Sansevierias being slow to react, it took it some time to show this leaf death. I have had this on my plants, show up a week after some insult to the plant, like cold spell, drying out, or maybe I never found out. I would continue to care for them as normal, but I do think moving air around them is often overlooked, and when they are stressed, it can make a big difference in recovery. for the record, I water my plants from above and get water into the cups and nothing bad happens.

Thanks everyone! I've cut out the affected area as gently as I could with a sterilized knife. The plant is also moved closer to a North window for some fresh air and sun. Other leafs do have vertical splits but don't show any sign of fungus so far.
Should I also use any specific fungicide for sans which could help at this point? Or would it be safer to wipe the leaves with some alcohol/hydrogen peroxide solution?

The tips may dry up or get sun burned from the top, make sure you water correctly, try watering them from the bottom, or growing it in more shade. It also may need a change of soil. There are many reasons it could be doing this doing this. You will be getting a lot of feed back. Norma