In the Malayalam script there are two ways to kill implicit vowels. The most widely applicable is the chandrakkala diacritic, similar to the virama that appears in other Brahmic scripts. There are also chillaksharam, modified forms of a small subset of consonants. Interestingly enough, the current sounds of chillaksharam don't match the consonants they appear to represent, hinting that a phonetic shift occurred since their introduction.
Why do these two ways of killing implicit vowels coexist? Do the chillaksharam have a Dravidian origin, or do they come from distinct attempts to adapt Dravidian scripts to accommodate Sanskrit vocabulary?