It's great that you were keyed in to gender and endings, as so many who aren't sensitive to them from their own language miss that. And sure enough, for grandis, you would need to make it neuter: grande malum, a perfectly fine phrase that is attested in Classical Latin.
Another way would be to use magnum (think "Pompey the Great" = Pompeius Magnus), and sure enough magnum malum is really common as well.
You should be aware though that malum can also be "a bad thing," and that "evil" in English used to be synonymous with "bad", but has a more sinister tone to it. While I think a Roman writer wouldn't have batted an eye at magnum malum, for your game, you may want to consider something else.
One way to do that is to turning magnum or grande into a superlative: maximum malum (or grandissimum malum, but with a heavy preference on the former for all sorts of reasons).
You could also choose a different word for evil. In particular, you have scelus (wickedness, crime, evil deed), which could work, or nefas, which is more classical than Medieval, I believe.
Both are nouns properly and are neuter, so you wouldn't need to change the accompanying adjectives.
However, if this "great evil" is a person, you might have to opt for Malus (masc.) or Mala (fem.), and the endings for the adjectives would be changed to -us and -a for magn-, maxim- or -is for grand-, respectively.
Answer from cmw on Stack ExchangeIn looking up the word "malus", I found that it can mean "evil" or "apple" or all "an upright pole, or mast".
Are these words just homophones, or is there more subtle etymology that caused these ideas to be related?
The various meanings of the word "malus" in Latin.
How to properly translate "Great Evil" into medieval Latin? - Latin Language Stack Exchange
Original meaning of 'malum' - Learning Latin - Textkit Greek and Latin
"Protect Me From Evil" - servo mihi ex malum / contego de m?
It's great that you were keyed in to gender and endings, as so many who aren't sensitive to them from their own language miss that. And sure enough, for grandis, you would need to make it neuter: grande malum, a perfectly fine phrase that is attested in Classical Latin.
Another way would be to use magnum (think "Pompey the Great" = Pompeius Magnus), and sure enough magnum malum is really common as well.
You should be aware though that malum can also be "a bad thing," and that "evil" in English used to be synonymous with "bad", but has a more sinister tone to it. While I think a Roman writer wouldn't have batted an eye at magnum malum, for your game, you may want to consider something else.
One way to do that is to turning magnum or grande into a superlative: maximum malum (or grandissimum malum, but with a heavy preference on the former for all sorts of reasons).
You could also choose a different word for evil. In particular, you have scelus (wickedness, crime, evil deed), which could work, or nefas, which is more classical than Medieval, I believe.
Both are nouns properly and are neuter, so you wouldn't need to change the accompanying adjectives.
However, if this "great evil" is a person, you might have to opt for Malus (masc.) or Mala (fem.), and the endings for the adjectives would be changed to -us and -a for magn-, maxim- or -is for grand-, respectively.
The Vulgata at Ieremias 32:42-44 has grande malum (in accusative, but for the neuter the nominative is the same). Many English translations translate that in great evil (e.g., King James).
The whole sentence is:
> Quia haec dicit Dominus: Sicut adduxi super populum istum omne malum hoc grande, sic adducam super eos omne bonum quod ego loquor ad eos.
The nominative neuter is either grande malum or malum grande, the former feels more medieval.
The malum grandis you found feels incorrect.
This is a full table of declensions:
| Number | Singular | Singular | Singular | Plural | Plural | Plural |
|---|---|---|---|---|---|---|
| Case / Gender | Masculine | Feminine | Neuter | Masculine | Feminine | Neuter |
| Nominative | Grandis Malus | Grandis Mala | Grande Malum | Grandes Mali | Grandes Malae | Grandia Mala |
| Genitive | Grandis Mali | Grandis Malae | Grandis Mali | Grandium Malorum | Grandium Malarum | Grandium Malorum |
| Dative | Grandi Malo | Grandi Malae | Grandi Malo | Grandibus Malis | Grandibus Malis | Grandibus Malis |
| Accusative | Grandem Malum | Grandem Malam | Grande Malum | Grandes Malos Grandis Malos |
Grandes Malas Grandis Malas |
Grandia Mala |
| Ablative | Grandi Malo | Grandi Mala | Grandi Malo | Grandibus Malis | Grandibus Malis | Grandibus Malis |
| Vocative | Grandis Male | Grandis Mala | Grande Malum | Grandes Mali | Grandes Malae | Grandia Mala |
| Locative | Grandi Malo | Grandi Mala | Grandi Malo | Grandibus Malis | Grandibus Malis | Grandibus Malis |