A challenging explorative deep high-pressure/high-temperature (HP/HT) well was planned and executed in the Mediterranean area of offshore Egypt. The expected bottomhole static temperature (BHST) and bottomhole pressure (BHP) were 400°F and 21,000 psi, respectively. To fulfill the requirements, a customized fluid was necessary during the planning phase. The main target was to achieve mud thermal stability. It was necessary to ensure longer mud exposure time during the extensive planned well-data acquisition of the explorative well to be drilled. Extensive laboratory work was required to develop a mud formulation with the lowest mud rheology. The aim was to build a suitable hydraulic model and manage equivalent circulating density (ECD) during the execution phase to achieve the smallest ECD impact. The drilling fluid was designed to help minimize pressure spikes resulting from the nature of the gel strengths when initiating circulation after downtime for connection for trips. Another fluid challenge was avoiding barite sag under HP/HT conditions to mitigate well-control issues during execution. During the planning phase, several laboratory tests were conducted using different base oils available in the market to deliver the best rheological model in accordance with the expected bottomhole temperature (BHT) and to achieve longer mud thermal stability, up to 168 hr static condition, at 392°F. The customization of the proper combination of high-performance additives and invert emulsion mud formulations was designed for a suitable fluid hydraulic model for each hole section, helping minimize the ECD in dynamic conditions as well as induced losses as potential causes of well-control issues. Drilling fluid, pressure, and temperature behavior were optimized during the planning phase. During the execution phase, over 516 days, operations never stopped to condition the fluid, and zero non-productive time (NPT) was recorded, confirming expectations under downhole conditions. All casing ran smoothly to the target depths; moreover, intensive logging programs, running nine days each, proved highly successful, including pressure points and fluid sampling.