This food looked horrible, (and) even grotesque.
In this sentence, do I have to put the word and before phrase even grotesque? And if I don't have to put the word and, what is the rule that allows me to delete the word and?
This food looked horrible, (and) even grotesque.
In this sentence, do I have to put the word and before phrase even grotesque? And if I don't have to put the word and, what is the rule that allows me to delete the word and?
An artful way to delete it is to rephrase using the adjective suffix ly.
This food looked horribly grotesque.
The literal meaning is slightly different but the spirit is the same.
Semicolons aren't all that common but they can join two independent clauses and show that they are strongly related.
This food looked horrible; in fact, it even looked grotesque. QaDT
If you want to see some masterful use of semicolons read some of Martin Luther Kings speeches.
Some prefer to avoid semicolons. In this case a simple comma can rid you of the and.
This food looked horrible, even grotesque.
When spoken it leads you to pause briefly.
A comma can separate two adjectives when the adjectives are interchangeable.
Grammar rules for lists allow the article separator, and, to be replaced with a comma. The comma "stands in" for the word and, replacing it.
This is half way between a comment and an answer, but I'll state it here. You could use grotesque even as an adjunct. More specifically as a supplement. This is a bit like putting the same phrase in brackets:
This food looked horrible, grotesque even.
In your example, 'even' is an adverb used as an intensifier (aka intensive) and meaning 'indeed'. The "and" is a red herring.
even
....
adv.
- .... b. Indeed; moreover. Used as an intensive: He was happy, even ecstatic.
(from The Free Dictionary)
Note the close parallel between your example and the example in the dictionary.
The "and" in your example is superfluous. In the interest of expressive economy, leave it out.
No prescriptive rule covers the case, because 'even' is (already) the connector; however (indeed, moreover), you might well ask what prescriptive rule allows the insertion of 'and' before a connective adverbial intensifier.
As for a descriptive "rule" covering the case, the topic would be more appropriate on a linguistics forum. This is beyond my expertise, but my understanding is that the descriptive "rule" ('rule' is used in a figurative technical sense here) would describe how a hypothesized prototypical or idealized linguistic deletion results in the use of connective adverbs without an 'and' component in a specified linguistic environment.
I would just add to what @CandiedOrange said that the main use of semicolons is to join two independent clauses, not clauses and phrases. Replace the semicolon with a comma and it's perfect -- two descriptive words in apposition to each other (horrible/grotesque), joined by a comma.