We report on the solar and interplanetary (IP) causes of the third largest geomagnetic storm (2018 August 26) in solar cycle 24. The 2018 August 20 coronal mass ejection (CME) originating from a quiescent filament region becomes a magnetic cloud (MC) at 1 au after ~5 days. The CME accelerates for about a day as evidenced by the time evolution of the CME speed, the intensity of the associated post-eruption arcade, and the reconnected flux. The presence of multiple coronal holes near the filament channel and the high-speed wind from them seem to have the combined effect of producing complex rotation in the corona and IP medium resulting in a high-inclination MC. The Dst time profile in the main phase steepens significantly (rapid increase in storm intensity) coincident with the density increase in the MC second half of the MC. We confirm that the enhanced strength of the 2018 August 26 storm is a direct result of the enhanced MC density by comparing with two other events: the 2010 May 28 MC that has similar properties except for no density enhancement and the 2014 April 11 MC with a complex density structure. The Comprehensive Inner Magnetosphere-Ionosphere (CIMI) model applied to the three events confirm that higher ring current energy results from larger dynamic pressure in the MCs. A complex temporal structure develops in the storm main phase if the underlying MC has a complex density structure during intervals of southward interplanetary magnetic field.