Learn Farsi: این طوفان رعد و برق هولناکی داشت.

In Farsi, the fixed pair رعد و برق (literally 'thunder and lightning') is the most natural and common way to refer to lightning during a storm. While you can technically say just برق (lightning), native speakers almost always use the paired phrase for weather phenomena, even if they are only emphasizing the visible lightning.

What is the ی doing at the very end of the adjective in هولناکی?

That is the indefinite suffix ی- (-i), acting like 'a' or 'some' in English. A crucial rule in Farsi is that when a noun is modified by an adjective, this indefinite suffix attaches to the very end of the entire noun phrase. So instead of putting it on the noun, رعد و برق هولناک (terrifying lightning) takes the suffix at the end to become رعد و برق هولناکی (a terrifying lightning).

How would we change this sentence to say 'Even this storm had terrifying lightning' using this lesson's grammar?

You would place the emphatic particle حتی (hattâ) right before the focused noun phrase ('this storm'), and pair it with the additive particle هم (ham) immediately after it. The sentence becomes: حتی این طوفان هم رعد و برق هولناکی داشت.

If we want to emphasize the lightning instead ('This storm had EVEN terrifying lightning'), where do the focus particles go?

To focus on the lightning rather than the storm, you bracket that specific part of the sentence. It becomes: این طوفان حتی رعد و برق هولناکی هم داشت. The goes immediately before what is surprising (the terrifying lightning), and the follows that entire noun phrase, right before the verb.