There is, as Richard points out, a whole Wikipedia page dedicated to the subject which is far superior to my quick answer down here.
Topheth is probably synonymous with the valley of Hinnom (Jer 7:32), which is exactly that valley south of Jerusalem where children was sacrificed to Moloech. So you are right when associating this with Gehenna/Hell. The horrible way they sacrificed the children was by "frying" them in the hands of a bronze statue. See this Wikipedia article
. That knowledge helps when trying to understand this text.
The king that should be punished is the Assyrian king.
From the notes of NET Bible:
The meaning of תָּפְתֶּה (tafÿteh), which occurs
only here, is unknown. The translation above (as with most English
versions) assumes an emendation to תֹּפֶת (tofet, "Topheth"; cf. NASB,
NIV, NLT) and places the final hey (ה) on the beginning of the next
word as an interrogative particle. Topheth was a place near Jerusalem
used as a burial ground (see Jer 7:32; 19:11).
From Zondervan Study Bible:
Topeth. A region outside Jerusalem where children were sacrificed to Molech (see 2Ki23:10; Jer 7:31-32; 19:6, 11-14 and notes), the god of
the Ammonites (see 1Ki 11:8). Thus it was a place of burning.
From Strongs Hebrew:
8611 topheth to'-feth from the base of 8608; a smiting, i.e.
(figuratively) contempt:--tabret. see HEBREW for 08608
8612 Topheth to'-feth the same as 8611; Topheth, a place near
Jerusalem:--Tophet, Topheth. see HEBREW for 08611