What is the contested ability modifier for a 0 Intelligence?

After an intellect devourer reduces a target's Intelligence score to 0, it can attempt to consume the target's brain with a contested check.

When this scenario came up during a recent Adventurers League game, I gave the affected player a -6 modifier on the contested check. That was a DM judgment call. I've verified that abilities with a 1 score have a -5 ability modifier in the PHB, but 0 is not on the chart.

An Intelligence Score of 0 or 1 gives a modifier of -5.

We know every even number is a + or - depending on which way above or below the base of 10 we go, so it would look like this:

\begin{array}{|c|c|} \text{Score} & \hline \text{Modifier} \\ \hline 20 &+5 \\\hline 18-19&+4\\\hline 16-17&+3\\\hline 14-15&+2\\\hline 12-13&+1\\\hline 10-11&0\\\hline 8-9&-1\\\hline 6-7&-2\\\hline 4-5&-3\\\hline 2-3&-4\\\hline 0-1&-5\\\hline \end{array}

Note that -6 does not happen unless the score somehow drops to a number below 0. If we follow the chart lower, then -1 or -2 would give a -6 modifier, -3 or -4 would give a -7 modifier, and so forth. Same with scores above 21. 22 and 23 grant a +6 modifier. Your judgement call was in error, but that's why it's a DM judgement call. Sometimes the answers do not readily present themselves. No harm done, and now you know better.

• Thankfully I didn't kill anyone, the player succeeded on the contested check and then the party wiped out the devourers. This chart is great for reference, thank you! Dec 6, 2016 at 2:59
• Alternatively, without a chart, you can use (score-10)/2, rounded down to get the modifier. Dec 6, 2016 at 10:14

The ability modifier for a score of 0 is −5.

This can be seen in the text immediately following the chart you reference, at the top of the next column (PHB p. 173):

To determine an ability modifier without consulting the table, subtract 10 from the ability score and then divide the total by 2 (round down).

In your case, (0−10)/2 = -5.