The Arabic word Allāh "God" is notable for a few different features. For one, it contains the sound [ɫ] not found in any other Arabic word; it's also an irregular contraction of the article al- and the noun ilāh "deity" (the expected form would be al-'ilāh, and this is indeed what's used for "the deity" in general).
Do we know when, or possibly even why, this change happened? Normally I associate brand-new unique phones with foreign borrowings (like Swahili having [ð] only in Arabic-derived words, or English using [x] only in loans), but Allāh seems to have a pure Arabic etymology. And I don't know any other words that have contracted with their article in this way.
P.S. The article does regularly assimilate before coronal consonants: al-šams "the sun" > aš-šams. That's not the phenomenon I'm interested in here: ilah doesn't start with a coronal consonant, so the assimilation doesn't happen. But it might be related if e.g. the assimilation used to happen before non-coronals too, making Allāh an archaicism.
EDIT: This question is similar, but doesn't explain the contraction. One of the answers implies that the unique pronunciation is to avoid confusion with the name of the deity Allāt, for example, but also says that the contraction is to glorify the name of God—if Allāt is also contracted, does that mean this contraction happens in all divine names? The line "almost all Arabian tribes were not monotheistic but they knew that 'Allah' is 'The authentic God'" seems to contradict that.