Learn Dutch: Hij zal vast iets raars achterlaten. - He will surely leave something weird behind.

While zullen often translates to "will" for the future tense, here it expresses a strong assumption or probability. Combined with the adverb vast (surely / certainly), Hij zal vast... means "He will surely..." or "I strongly assume he is going to...".

Why does raar have an -s at the end in iets raars?

When you use words like iets (something), niets (nothing), or veel (much) followed by an adjective, that adjective gets an -s added to the end. So, iets raars means "something weird".

Is achterlaten a separable verb? Why isn't it split up in this sentence?

Yes, achterlaten (to leave behind) is a separable verb made of achter + laten. However, because the modal verb zal takes the conjugated second position in the sentence, the main verb achterlaten is pushed to the very end as a full, attached infinitive.

Could I use zeker instead of vast in this sentence?