That revolting creature made a promise, you ask?
Here it is:
“(Iblees) said: “Because You have sent me astray, surely I will sit in wait against them (human beings) on Your Straight Path. Then I will come to them from before them and behind them, from their right and from their left, and You will not find most of them as thankful ones (i.e. they will not be dutiful to You).” [Surah Al-Araaf (7) : 17]
[Notice how he blames Allah for misleading him, rather than blaming himself. This was the difference between Shaytaan and Adam (alaihissalam). The former blamed another and therefore refused to change while the latter blamed himself and repented, thereby becoming a better person.]
So, what does he mean by coming from the right, left, etc?
Here’s an explanation of these two verses from Tafsir Ibn Kathir and Tafsir As-Saadi.
For those who wish to learn more about Shaytaan’s way of fooling us, you may find the following books/articles beneficial, insha-Allah:
[Don’t feel overwhelmed. You don’t have to read them all. Just read one for now so that you get an idea of how Shaytaan does business.]
1. Shaytaan (Satan) and His Ways of Approaching the Believers by Abdullah Al-Khater
2. Shaytaan: His Ways in Misleading by Abu Iyaad
3. The Methods of Shaytaan in Leading People Astray by Shaikh Abu Islam Salih Ibn Taha Abdul Wahid
Also, here are a whole bunch of questions about Shaytaan from Islam QA.